Tube, patch kit, tire levers, hex-key/screwdriver tool, and $20 bill (this item gets used most often). I have a pump on the bike as well. I have a bag on every bike I own (it's worth it not to have to worry that I'll have forgotten it when I most need it).

Cell phone depends on where I'm riding (there are a lot of places around here where a cell phone is just dead weight).