Pockets:
-Cell phone
-wallet (if ride is over 15 miles)
-chapstick
-sometimes a small digital camera

Seat bag:
-gum
-one tube in a ziploc bag
-my cute teeny box of glueless patches
-two tire levers
-2 c02 cartridges and a teeny weeny c02 pump
-multitool
-alcohol wipes in case of scrapes (they don't take up any room really)
-1-3 individually wrapped "wet wipes" for cleaning up hands after changing flats

I will usually fit in a snack (luna bar, sportbeans) somewhere for rides over 30 miles.