Extra zoom lens - usually wide (17-70) because my normal (24-120) is on the camera, and the LensBaby. Lens cloth, extra batt, charger, various filters, on-camera flash (though I HATE to use it!), extra cards, some form of snack (cereal bars are a fav), and usually the reason I'm using my bag (I usually shoot in studio) is because I'll either meet or be with a client, so I carry a few different kinds of breath mints.

Don't ever let your lunch of saurkraut and onions ruin a starting relationship with a new client! Got the idea from assisting - one of the shoots, the client (wasn't the photographer) had something horrible for breakfast and his breath stunk up the room - it really travelled, and we couldn't get away from it!
The photographer then showed me his breath mints and winked ;-)

-Lew ;-)