Primer is a low-budget science fiction film from 2004 which has become a cult film in sci-fi circles. It’s about two engineers who invent a time machine that they use to make money in the stock market.
After a while, things start to go wrong and they try to change the present by changing the past in a very complex way. I’ve been into these kind of stories ever since Back to the Future. I’m also very amused by this timeline analysis with nine different parallel universes that someone has put on the web.