Legendary filmmaker/exhibitor Jonas Mekas has put dozens (if not hundreds) of his films for sale online. All the files are mp4 -- suitable for your video iPod, computer, etc. Currently Mekas is undertaking a project in which he will make a short film a day, every day this year. Each day you can download the movie for free; after that, you have to pay for it. A fantastic idea.
A dig through the entire site reveals all sorts of interesting stuff -- Kenneth Anger movies, outtakes from Tarkovsky's The Sacrifice... Enjoy!