Primer. You really have to see it. If you liked Pi, this one's a corker. Here's the blurb from its Wiki page; a nice synopsis 'cos I'm lazy;
"Primer (2004) is an American science fiction film about the accidental invention of time travel. The film was written, directed and produced by Shane Carruth, a former mathematician and engineer, and was completed on a budget of only $7,000.
The film stars Carruth as Aaron and David Sullivan as Abe, two engineers who create a device which will allow an object or person to travel backwards in time. The pair initially use the device to cheat on the stock market, but ultimately they cannot resist the temptation to meddle with every aspect of their lives. Through recklessness they create increasingly complex paradoxes, and ultimately their newfound power begins to destroy their friendship.
Primer is of note for its extremely low budget, experimental plot structure and complex technical dialogue, which Carruth chose not to dumb-down for the sake of his audience. One reviewer said that "anybody who claims [to] fully understand what's going on in Primer after seeing it just once is either a savant or a liar." The film collected the Grand Jury Prize at Sundance in 2004 before securing a limited release in cinemas, and has since gained a cult following."
[Wikipedia](yes, yes I know...)
Its really hardcore to attempt following the separate time threads and paradoxes, it spawns a fair few with different versions of both characters moving through each other's worlds (timelines). I can't praise it highly enough.
For all its complexity, it is still a great watch.