http://www.mirrycle.com/

I use an STI-mounted Mirrycle, they also have models that mount to traditional brakes (brake line coming out of the top). Depends on your setup.

It's great. The initial adjustment is a bit finnicky, and the view can be partly blocked when I'm wearing a bulkier coat, but it's enough to provide the situational awareness that makes riding in the city much easier. My head will sometimes bob slightly to get the right angle, but I've never felt the need to steer to aim what I'm looking at.