I have two camera bags that are generally ready to go at all times.
Bag #1 is for work (and belongs to my employer): Canon 20D, 50mm f/2.5 macro, kit lens (18-55mm, I believe), Nikon SB-600, two batteries, two sets of flash batteries, extra quick release plate, one gig CF card in camera, extra 512 meg CF card. (I shoot nothing but small jpegs.) The bag is a medium-sized Canon bag. My Bogen 3036 always comes along as well, and so does my Sunpak 555 sometimes.
Bag #2 is for spur of the moment shooting, when I just need to grab something and go: Two unloaded Canon FD bodies (usually an old F-1 and either an FTb or an AE-1P), 55mm f/1.2, 28mm f/2.0. Two rolls HP5 or Tri-X (Arista Premium 400 at this time), two rolls Delta 3200, and two rolls Fuji Press 800 are always in the bag at room temperature, and I usually grab some cold film before I leave the house. The bag is a smallish toiletries bag (about 10x5x5 inches) with a side zipper pocket. All this stuff is crammed into it pretty tightly. Oh yeah! The most important thing: a SHARPIE!