Topeak Mini 6

This incredibly compact, bike-oriented multi-tool has five different sizes of Allen wrench plus a Phillips screwdriver head, all of which folds up into a little pod about the size of a walnut. Sometimes I'll carry it in my pocket or toss it in shoulder bag; mostly I keep it in the under-seat pouch of my bike. It really comes in handy for quick adjustments: raising the seat height, tightening the rear view mirror, adding and removing accessories, etc. Because it's so small it doesn't give enough leverage for really tight nuts (you can't remove a handlebar stem with it), but by extending the tools on the opposite side of the one you're using you can get a handle that's effectively 2.5 inches long, which is enough for small jobs. It also works well as a keychain fob, though at 58g it's slightly on the heavy side.