Timbuk2 -- Messenger Bags. Since we are on the topic, I've used dozens of bags and none quite compares in comfort to a bike messenger bag. I use a small one from Nike just about every day. It is just so much more comfortable than carry to have it go over one shoulder.
Timbuk2 even lets you build a custom bag. So a great combo is a messenger bag from timbuk2 and accessory stuff from waterfield.