In my messenger bag on a typical day:

Spare tube
Tire levers
U Lock
Cable
2 pens
1 pencil
A blinky red light for night time
Keys
2 dimes
1 quarter
Bra and underwear
Cleat covers
Various books
Sometimes an iBook
Glasses case
Stick of deodorant

It's been known to have:

Half a watermelon
A bag of apples
Jug of juicy juice
Cucumbers
Towels
Overnight kit
Hairbrush
Various clothing
Beer