You can read reviews from the bike helmet safety institute here
http://www.helmets.org/
Although all helmets have to pass the same test they recommend:
1. More rounded shape - less "snag" points.
2. More styrofoam = better protection
(but more vents = more comfortable, and therefore more likely to wear!)

The Bell Metro is a good commuter helmet - nice round shape and it has places for attaching things like lights and a snap on cover. I think the helmets people don't like the snaps to attach things as they think they are snag points but there is also the simpler Bell Citi.

The material can deteriorate in sunshine so you want to replace your helmets periodically (5 years? Though bike helmet manufacturers may say less)