I use 2 LED flashlights (each ~90-100 lumens) that run on rechargeable AA batteries. I use the "two fish lock blocks" to hold them -- very cheap and effective.

200 lumens is a bare minimum I'd say for commuting in the dark, but I'm usually with my wife and her bike has a very nice Ixon LED headlight. 3 of these flashlights would probably be sufficient, but I'm too lazy to rig up another one. Purpose-made lights for bikes are generally more expensive, but probably are more effective. I prefer battery powered lights regardless, but some folks really like hub generators. Some even still use tire generators.

Just my $0.02