Off the top of my head:
1. Why We Fight
Arguably the greatest political documentary of all time.
2. Fahrenheit 9/11
Michael Moore's masterpiece. This movie helped me get through the second Bush term.
3. Idiocracy
Palin + Bush + FNC + Teabaggers = Brawndo!
4. Thirteen Days
I could watch this with my hair on fire.
5. The Contender
President The Dude! And Gary Oldman! And shark steak!
6. Bowling for Columbine
Michael Moore's other masterpiece.
7. SiCKO
Michael Moore's other masterpiece.
8. The Fog of War
Don't watch this right after watching Why We Fight. Your brain will melt.
9. Food, Inc.
The emerging crisis of this century.
10. The Corporation
Just really goddamn scary.
Also, Walmart: The High Cost of Low Prices and Who Killed the Electric Car. Any other really great ones I missed?