Primer (2004)
Bending its celluloid into a Möbius strip, this miraculously low-budget (only $7,000) sci-fi thinker about a pair of friends who accidentally discover time-travel has a labyrinthine, paradox-studded plot that works like a treadmill for your brain. Luckily it's also fun, so you won't mind when you inevitably have to rewatch it. Keith Staskiewicz