The Lowepro SlingShot shoulder bag, which is slung over one shoulder but can be spun around to your chest for easy camera access, got high marks from Bill Howard over at Gearlog:
The SlingShot is the most convenient camera bag I've used in the field. Buckles keep the zipper from unzipping too far and spilling the contents; a rain shell folds out from the bottom.
Lowepro offers the bags in three sizes and prices ($70, $90, $110), depending on how much kit you bring along with your camera—the only color is black.
I use a small over-the-shoulder bag for my camcorder sometimes. It really is great to be able to get at it without taking off my bag, especially in crowded settings.