Okay, I'll try to start us off with one:
Primer (2004) sci-fi drama about time travel. It's got an absolutely brain melting plot, so if you're in the mood for some mindfuckery, give it a look.
Also features a very interesting and realistic time-travel concept.
It's a short movie too, so won't take up much time to watch, it clocks in at just 72 minutes long.
Fun fact: the movie was made on a budget of just $7,000, that's less than half as much as Paranormal Activity. Sure it didn't gross anywhere near as much, but it looks a hell of a lot better.