I've seen this done several times in different places and decided to try it myself.
Mostly because I know I carry a lot of crap in my bag.
This stuff lives in "Big Yellow" my Ortlieb water proof messenger bag
*Rain jacket - Rain pants - small towel - Arm warmers - T-shirt
*KC street map (just in case)
*Tubes (2 sizes)
*Blackburn pump with bag to keep it clean and "safe"
*Tool bag - 15mm wrench - C02 cartridges/shooter - tire levers - spoke wrench - Patch kit - multi tool
*Food bag - Clif bars, gum, hand warmers
*Camera bag w/camera, batteries, charger, cords etc
*Mini tape recorder w/spare cassette and batteries (started carrying this some time back for some reason. Never has been used)
*Note pad with pen - for scribbling my random thoughts
*Travel tooth brush, Red Fang beer coozie, Trojan, Sharpie
No wonder my bag is so heavy when its "empty"