Shane Carruth, writer and director of the lo-fi scifi flick Primer, has uploaded the entire film to Google Video so you can watch it for free. If you liked the time-twists of Inception, you'll love Primer - it's even crazier.
Carruth wrote, directed and starred in this understated, weird movie that came out in 2004. It's the most realistic depiction of time travel you're ever likely to see, and it leads to a temporal paradox that gets more interesting the more you think about it.
UPDATE: Sorry folks - looks like Carruth has had a change of heart and taken the film down. You can find out more about the movie, or order a copy, on the official Primer website.
Yes, it's a serious brain-twister. Need help? Don't mind minor spoilers? Try this handy chart: