I agree with alot of the uniforms mentioned here already, but it's so hard to narrow it down. There are some really cool uniforms in football, and not just in the NFL either. Here's ten of my favorites:

1980 Philadelphia Eagles home green jersey, silver-grey pants
1979 Houston Oilers home light blue jersey, white pants
1978 Denver Broncos home orange jersey, white pants
1983 USFL Houston Gamblers home black jersey, silver-grey pants
2001 XFL Birmingham Bolts home purple jersey, white pants
2006 ArenaFootball San Jose Sabrecats green jersey uniform
1994 San Francisco 49ers "throwback" uniforms
1994 Dallas Cowboys "throwback" uniforms
1999 WorldLeague Barcelona Dragons green jersey uniforms
2005 ArenaFootball New York Dragons black jersey uniforms