I use this mirror with my road bike. After a small learning curve I love it and hate to ride without it. It is amazing how much you can see behind you with such a small thing.
http://www.teamestrogen.com/products.asp?pID=4043
With my old mountian bike I liked using a handlebar end mounted mirror. I couldn't quite figure out how to put a handlebar mirror on my road bike to get good visibility.