"After enduring one of the worst slumps on record, we're beginning to see a rebound in all major commodities market," says Lee Lowell, adding "But one commodity stands out in particular -- natural gas."

In Xcelerated Profits Report, he explains, "This could represent the best buying opportunity in several years." Here's his review of the United States Natural Gas Fund (NYSE: UNG).

"As it so often does, it's no surprise to see the energy market leading commodities higher over the past few weeks -- specifically, crude oil and natural gas.

"It's hard to find any good news these days but I was pleasantly surprised with the third-quarter railway results, as almost all of the 'class 1 carriers' reported better than expected earnings," notes analyst Tom Slee.

The contributing editor to Gordon Pape's Internet Wealth Builder explains, "Several rail stocks are starting to look attractive at these depressed levels and Burlington Northern (NYSE: BNI) remains my preferred choice in the group." Here's his outlook.

"Even with the economic downturn starting to bite, reduced fuel costs and increased freight rates offset lower volumes. Equally important, the companies remain cautiously optimistic despite the miserable outlook.

"They are confident that further freight rate price increases in the 4% to 5% range are sustainable and will still allow them to undercut inefficient truckers.

"Unfortunately, none of this prevented the stocks from being battered during the market collapse. However, I think that fourth-quarter profits are likely to remain strong and the longer term outlook for railroads remains favorable.

"Burlington Northern continues to power ahead. A shrinking economy must eventually take its toll but there was no sign of any weakness in BNI's third-quarter results. Operating earnings came in at $1.91 a share, up 29% from $1.48 in 2007.

"Gold is now looking stronger; it is time that investors have gold in their portfolios," says Curtis Hesler. In the The Professional Timing Service, he looks at gold's seasonal patterns.

"I think they will rush to commodity-based assets because of the serious underinvestment phase the commodity sector is involved in now. This will lead to shortages and very high prices down the road in all commodities.

"Once the dollar begins to roll over, gold will be an instant benefactor. It is already looking stronger in my technical work, and it is time that investors should have gold in their portfolios. I still recommend that you put new money into the major gold miners only.

"We are approaching an interesting seasonal period for gold. Years ago, the Stock Trader's Almanac used to specify a seasonal trade in gold.

"Their study showed that if you bought ASA Ltd. (NYSE: ASA) at its low in November and sold it at its high in the first quarter of the next year, you would have averaged a gain of 87.8%.

"Crude oil remains deeply oversold on an intermediate-term basis, suggesting a rally sometime in the early fall," says Dennis Slothower.

The editor of Stealth Stocks looks to Petroleo Brasileiro SA (NYSE: PBR) as his latest "stock of the month." Here's his review of the Brazilian firm that is now the world's 8th-largest oil company.

"The founding of Petrobras was authorized in October 1953, with the objective of executing, on behalf of the federal government, the activities of the oil sector in Brazil.

"Over the past five decades or so, the company has become the country's leader in the distribution of oil products, an activity not covered by the government monopoly, and today it is internationally acknowledged as the eighth-largest oil company in the world.

"Leading the sector in the development of one of the most advanced deepwater and ultra-deepwater technologies for oil production, PBR was twice (in 1992 and 2001) awarded the Offshore Technology Company (OTC) prize, the most important award in the sector.

"Railroads are a play on three big secular themes: the drive for increased energy efficiency, growth in coal and the agriculture boom," says Elliott Gue, a energy sector expert who has just returned from Japan where he was covering the G8 Summit.

Meanwhile, in his The Energy Srategist, he states, "Railroads are now among the most fuel-efficient forms of freight transport available." Here, he offers a bullish review of Union Pacific (NYSE: UNP).

"My long-held thesis on the group has been that the railroads are no longer totally dependent on the US economy for their growth.

"It's no longer appropriate to look at this sector as viciously economy sensitive. The traditional relationship between the broader market and the rails has been breaking down for several years, but this trend appears to be accelerating.

"In 2007, according to the Association of American Railroads (AAR), the average railroad moved a ton of freight a distance of 436 miles on a single gallon of diesel fuel. That makes freight trains roughly three to four times more fuel efficient than trucks.

"Union Pacific is the largest railroad in the US and has long been one of my favorites. The company's network is nearly 33,000 miles long and is concentrated in the West and Midwest. It also offers a convenient example of the bullish forces at work for the rails, particularly in the coal and agriculture industries.

"As steel prices continue to climb, one company that is set to profit handsomely is Cleveland-Cliffs (NYSE: CLF)," says Bill Martin.

Adding to the stock's appeal, the editor of BullMarket.com explains, "Event-driven hedge fund Harbinger Capital has been an aggressive buyer of the stock." Here's his review of the situation.

"Shares of Cleveland-Cliffs have been on fire, up over 150% year over year and they have more than doubled year to date. The Cleveland, Ohio-based company is the largest producer of iron ore pellets in North America and a major supplier of metallurgical coal to the global steel-making industry.

"Cleveland-Cliffs benchmarks iron ore prices to the price of steel, so when steel prices rise, so do iron ore prices. The company said all of its North American iron ore mines are producing at or near capacity.

"Cleveland-Cliffs ended the first quarter of 2008 with $186.5 million of cash and cash equivalents and $600 million in borrowings outstanding under an $800 million credit facility. The company expects to generate approximately $700 million in cash from operations in FY08 as it sells through its inventory.

"Event-driven hedge fund Harbinger Capital was an aggressive buyer of the stock in May, paying between $76.96 to $104.75 a share to add to its position in the name. For the month, the firm spent approximately $338.5 million to acquire nearly 3.7 million shares.

"The number one reason I like gold is because of inflation -- now a big problem in the emerging markets and the major economies," says resource expert Eric Roseman.

In his industry-leading Commodity Trend Alert, he says, "One of my favorite companies in the world is Goldcorp (NYSE: GG)." Here, he looks at this gold mining firm.

"Inflation sits at a nine-and-a-half-year high in Asia at 7.5%, a 15-year high in the Euro-zone at 3.7% and in the United States it's at 4.2% -- if you believe government data in the first place. I don't. I say inflation is running closer to 10% in 2008, not 4.2%.

"The cost of living, mainly in food and energy, is now totally out of control and destroying business margins and eroding the purchasing power of consumers, especially in the emerging markets where food and energy consumption devours more than 65% of wages.

"It seems very obvious to me that Asian governments have now lost control of inflation. The same applies to the Gulf countries which peg their currencies to the dollar. And in Europe, the European Central Bank is freaking out because of high inflation.

Although he has been maintaining a cautious stance on the refining group, energy sector expert Elliott Gue is now boosting the rating on Valero Energy (NYSE: VLO).

In his The Energy Strategist, the advisor explains, "Valero is now attractive for three reasons: superior geographic exposure, refinery complexity and a new focus on profitability."

"Our caution on the refining group was due to expectations that crack spreads would be weak through the spring, a period during which spreads tend to widen. Overall, this call was correct: Refiners have underperformed the energy patch since mid-March.

"And longer term, I have some concerns about new refining capacity expansions due to come online over the next few years. As this supply comes online, it could put downside pressure on margins.

"But over the next six to nine months, the refiners look like a compelling play. Gasoline inventories are now back in line with seasonal norms; it's likely gasoline prices will now rally further relative to crude oil. In fact, we're already seeing an obvious spike in crack spreads.

Leo Fasciocco is a technical expert who focuses exclusively on finding breakout candidates. In his Ticker Tape Digest, he looks to agriculture equipment manufacturer Deere & Co. (NYSE: DE).

"Deere, with annual revenues of $24.8 billion, makes agricultural, industrial, forestry, and lawn-care equipment. DE is benefiting from the strong demand for its products in the farm sector. The company is also expanding aggressively in Russia.

"DE has broken out from a 13-week flat base. Its long-term chart shows DE soaring from 20 in 2003 - the start of the prior bull market - to 92. It has been a big winner in the big cap sector. The stock has gained 65% in the past 12 months versus a 5% drop in the S&P 500 index.

"The stock is in a base bracketed between roughly 78 on the downside and 91 on the upside. The breakpoint was set at the key upside resistance of the base. Deere has plowed through that resistance on increasing volume.

"Green investing and clean energy may be the politically correct topic at cocktail parties, but coal is the economically correct vehicle for investors," says Ronald Rowland and Brandon Clay.

The editors of All Star Investor explain, "Coal has been an energy source for millennia -- and is still the number #1 source of energy for electric power plants in the world." And, they add, "One of the best places to invest in coal is Consol Energy (NYSE: CNX).

"Prehistoric Chinese are said to have used coal for heating. According to Roman historians, Britain burned coal in the first century. Throughout history, coal has been the primary source of heat in homes.

"Rapidly industrializing nations like China are still dependent upon coal for energy. Overall global consumption has not diminished either. Coal fuels 48% of electricity plant generators. And the trend is heading upward – probably for the next 30 years. Despite the deafening rhetoric, coal is not going away anytime soon. Investors should take notice.

The New York Times recently made a case for investing in stocks to gain exposure to commodities:

But there is an easy way to get a shot at commodity-like returns, without investing directly in commodities or their indexes. Investors can buy shares of the natural resource companies that produce commodities.

However, investors need to be careful: just because a stock is in a sector related to commodities doesn't mean that it will move with the prices of those commodities. Other factors, including decisions by management, any hedging the company may have in place, and other company-specific factors, may cause their returns to differ from those of the underlying commodities. While it's quite true that, as the New York Times points out, natural resources stocks have outperformed commodities in recent years, that is not a trend that will necessarily continue.

There may be good reasons to buy these stocks, but if you want to make a bet on commodities prices, the best way to do that is with exchange-traded funds (ETFs). Back in his days at the Motley Fool, BloggingStocks contributor (who is the source for news and analysis for private equity, by the way) Tom Taulli wrote a nice piece about how to invest in commodities through ETFs.

If you really want to learn about ETFs, I recommend picking up a copy of Investing with Exchange-Traded Funds Made Easy: Higher Returns with Lower Costs -- Do It Yourself Strategies without Paying Fund Managers by Marvin Appel.