Tools: bike spanner, multitool, spare tube, preglued patches, tyre lever, pump
Medical: lipsalve, small amount of chamois cream, loo roll
Clothing: waterproof, spare shorts (I wear shorts, shirt, jersey, jacket, longs, gloves, and cap. Depending on weather, some of this will be in my saddlebag)
Other: batteries, mp3 player, lightweight lock, space blanket.
In my pockets: phone, cash, brevet card.
With that lot I'm warm enough over the summer at night, reasonably comfortable, and reasonably self-sufficient. If anything goes so badly wrong with me or the bike that I can't fix it with that lot, I wouldn't want to carry on anyway.
The lock is a comfort when you want to nip into shops or go to the loo. The space blanket makes sleeping in service stations and the like more comfortable. I'd like to be able to carry more clothing, but I have a carradice barley and that's its limit.