Like Regina, I always have my little seat bag with spare tube, patch kit, tire irons and multi-tool, plus pump bracketed to the frame (I have just enough room behind my seat tube for the pump).

For other stuff, I got a Madden Piggy Pack from Adventure Cycling. It holds cell phone, snack, keys, and bandanna, plus it has a removable map holder for a cue sheet. I tried a Bento Box, but my top tube is so short that I was bashing into the Bento Box every time I mounted or dismounted. Sold it to my neighbor who's about 6 feet tall--it's perfect for her.