FreeDocumentaries is a large repository of documentary films that have been released by their respective copyright owners for public display. You won't be stuck watching dated old films however - the offerings are FreeDocumentaries are recent and intriguing.
One of the problems with many of the free offerings you'll find on the internet, especially in the category of full length movies, is that they're rather lacking and often filled with outdated and boring films. FreeDocumentaries is packed with recent films that are quite interesting like Adam Curtis' The Century of the Self, Michael Winterbottom's The Road to Guantanamo, and Morgan Spurlock's Super Size Me.
You can browse the documentaries by topic, search by title or topic, or view films by the regions they document. Clicking on any film listing will give you a trailer, additional information about the film, and allow you to select the window size you want to watch the film in. FreeDocumentaries is a completely free and requires no registration. Have a favourite site for watching free movies online? Let's hear about it in the comments.