I use a Rapid Transit Recon 19 bag. It is pretty much a perfect size for most of the stuff I do, and I can even fit a weekend's worth of clothes in it (but I'm a guy :P). It has lots of compartments for just about anything, and I really like the attached rain cover, you can't forget it at home. It was $72 from
New Enough. They have more pictures of the bag there.
Here are a couple pictures from various rides.
Me in my fat man jacket, shows how it fits with a person on the bike.