My kit looks pretty much like most, I carry a small saddle bag with:

CO2 pump w/extra cartridge - small, light, and only for roadside emergencies. I used to carry a frame pump, but my bike now wont work properly with one, and most mini-pumps are garbage. If it takes me more than 2 cartridges to fix myself, I probably have more pressing issues than a flat tire.
Tire levers - I like the yellow Pedros levers, they are nice and wide and haven't broken yet.
New tube - I would carry a used tube, but I can never get it rolled as tightly as a new one fresh from a box, so a used one won't fit in my bag easily.
Patches - just in case. I prefer the Park Tools glueless patches. They are cheap, come in a tiny plastic box that protects them, and have a little piece of sandpaper rather than the metal scraper thing. Just sand the spot for a second, wipe it off, and stick a patch on. Super easy to use, and I have yet to see one fail.
Mini-tool - I think mine has 4 allen wrench sizes, flat and phillips screwdrivers, and a couple other things that I havent used yet.
Batteries for my lights - I made a point to make sure that my headlight and taillight took the same kind of battery, and I carry enough to replace one or the other.
During winter I also carry light gloves and a beanie, assuming that I don't leave already wearing them. I like using the fleece liners from a pair of snowboarding mittens - they keep me warm and pack easily.

On work days, I also carry a water bottle and a coffee travel cup. Gotta have my Joe!

With a single speed a pair of nitrile gloves would be a good idea, but my commuters all have deraillers, so I can get the wheel on and of pretty easily without getting too much grease on my hands.


Ultimately, I think the big factor to what you carry is going to depend on how far you ride everyday, and how far your route takes you from help. I dont carry nearly as much as some people here, but I am also never more than a 15 minute walk from either home, a train station (which the one closest to my office even has a bike shop for commuters), or a bike shop. If things get really jacked, I would be able to get help rather quickly.

If my commute took me over the hills and through the woods though, I would probably carry more stuff. Things like a chainbreaker with a couple extra links of chain, spoke wrench, and a frame pump start looking more practical.

If I was abandoning civilization and going on a self supported tour, I would carry an extra rear-derailler, extra spokes, a folding tire, 2-3 tubes and 2-3 patch kits, extra water, raingear, extra brake/shifter cables, etc.