I use a Domke ("The Orginal" $99 @ Calumet). Its black canvas and it doesn't look like a camara bag, which I like when I travel. I hate looking like a tourist. They even have a navy bag with tan trim that really makes it look like a small travel duffle.

Its large enough that I can carry 2 35mm bodies, one with a lens, 2 more lenses, my vivitar flash, batteries, plenty of film and extra space for keys and stuff... Or, I can make a small adjustment and have enough room for my Mamiya TLR, flash, light meter and probably enough room for 2 more lenes and all the other stuff.

They even have a larger version ($134 @ Calumet), which I am sure would be overkill for me.