Hub Bicycles in Five Points is a great place to pick up a part, accessory or a whole bicycle. The staff really knows their stuff, and they have a good selection of bikes to choose from. Plus they'll let you take a test drive to make sure the bike fits like you want it to. There are only a few bike shops in Athens, and this is the best one of the bunch. Their prices are fair for a local shop.
PROS: Good Selection, Friendly Staff, Local, Convenient