If you enjoy following the comings and goings of activist hedge funds like Third Point and Pirate Capital, you will definitely want to bookmark the StreetInsider.com 13D Tracker. Every time an investor acquires more than 5% of the outstanding shares of a publicly-traded company, they are required to file a Schedule 13D form with the Securities and Exchange Commission. In the form, you can find important information such as how many shares they acquired and what they plan to do with their shares (were they acquired just for investment or does the fund plan to lobby for changes in the business?).

Most exciting, some investors will attach an "open letter" to the company in the 13D filing. Dan Loeb of Third Point is most famous for his diatribes, including this gem to the CEO of Star Gas Partners, L.P (NYSE: SGU), Irik Sevin, a few years ago: "It is time for you to step down from your role as CEO and director so that you can do what you do best: retreat to your waterfront mansion in the Hampton's where you can play tennis and hobnob with your fellow socialites. The matter of repairing the mess you have created should be left to professional management and those that have an economic stake in the outcome." To read more about Dan Loeb, go here to download an excellent piece from Bloomberg Magazine.

While you can of course view the 13D filings yourself in the SEC's Edgar database, the 13D Tracker provides a concise overview of the most interesting ones, along with background information.