I carry the "usual" stuff in my under-seat bag, and for the past few years have been using a "Bento Box" too. It's a small "bag" that fits just behind the stem and rests on the top tube, so you can easily open it while riding. I keep money, ID, Gu, Kleenex, lip balm, aspirin in there -- anything that I either may want while riding or that I don't want to fish around for.

Here's a link: http://www.all3sports.com/product/ca.../ProductID/220
(Um, I haven't used all3sports, this is the only place I could find a good picture).