I always stole my bf's seat bag and would forget to put it back. So he would realize he was bagless halfway through his ride, oops. So he made me get my own.

I just bought the same one he has. Its the specialized mini wedgie. It fits a small pump, one tube, and tire levers. Takes a little stuffing and maneuvering, but it all gets in there. You can also stash some cash in a little pocket inside. It's only about $20 and just fits what you need in case of a flat.

He has an all carbon bike and swears by this little bag.