I had to give this at least a mention. Martin Scorsese's Taxi Driver (1976) is now free to watch on YouTube. If you live in the US, you can watch the classic film starring Robert DeNiro, Jodie Foster, and Harvey Keitel here. If you live outside the US, you're unfortunately out of luck, at least for now. (There's some hope that the film will get rolled out to international markets soon.) Normally, I wouldn't knowingly highlight a geo-blocked film -- it's not very "open," I know -- but this seemed a little too good to pass up. This YouTube promotion is sponsored by Crackle.com, a Sony Pictures site where you can watch other major films online.
NOTE: You can find many great films in our collection of Free Movies Online.