I carry a good tool kit, always has been used for someone else out on the road though. Also like Moe said, the tool you used last for putting something on the bike. Also a flare for the traffic, and a very well stocked first aid kit. The American red cross makes one that fits perfectly on bikes. I add a to it a bit, and it's definitely come in handy a few times.

Have a heart that never hardens, a temper that never tires, a touch that never hurts. - Charles Dickens