Minimum to bring on a ride....

What you should stick in that little bag under your seat (or in your jersey pocket)

-For the light weight minimalist:
Tube/Tire Levers/CO2 Cartridge/CO2 Cartridge Adapter

-For the one playing it safe:

The above mentioned plus:

Cell Phone/$20 Bill/Rain Jacket/Gel and or Power Bar/Water
Phone # of Precision Bikes for Road Side Assistance.