On our weekend ride it was wet and miserable and nearly everyone had troubles with flat tyres. This lead to a debate between everyone on who had the best and smartest emergency tookit set. You ask 10 different cyclists what they carry for tools on the road and you'll get 10 different answers. A few different points of view is always good and here is mine along with some suggestions from others.
1. spare tube. Don't leave home without it.
2. multi-tool. Don't leave home without it either.
3. tyre lever. Best to carry 2 just in case one breaks. Doesn't take much more room to do this.
4. $5 bill. This can come in handy for 2 things. Buying food in case of emergency bonk, or to fix a major tear in your tyre. See this post.
5. Patch Kit. Where there is one puncture, there are usually mutliples. This doesn't take up much room and one day far from home you'll be glad you have it.
5. CO2 Cartridge. These are great, but you have one shot to make it work. If you screw it up you'll be stranded. See points 6 & 7
6. Valve converter. This comes in handy when you're near a service station and either don't want to use your CO2, or if you've messed up the first try with your CO2.
7. Crowie made a great point about carrying a hand pump. Many people mess up the fitting of the tyre/tube when fixing it when in a rush. They also don't check properly for peices of glass or debris and end up with another puncture moments after they fix it. Using a CO2 cartridge is quick and easy which enables you to be in a rush. Using a hand pump that requires a lot of elbow grease will make you damn sure that the tube is fitted properly and all the debris is out of the tyre before you take 5 minutes to pump that thing up to 100psi.