I like the Timbuk2 bike seat bags. I have the medium sized one and it holds a tube, CO2 cartridge and head, a couple of allen keys, a tire boot and my house keys no problem. The velcro that goes around the seatpost was a bit long for my 27.2 mm diameter post, but I just trimmed off the excess.
http://www.timbuk2.com/tb2/products/...bike-seat-pack