Topeak Alien II multi-tool.
It has various allen wrenches, open and closed-end wrenches, philips screw driver, flat screwdriver, the requisite bottle opener, tire removal tabs, chain splitter, and of course a knife. It's reasonably small, and accommodates most of my needs. Actually I find it pretty convenient to use even for minor jobs in my own garage (rather than hauling out my full-sized tools).
All it lacks are pliers, which are of limited usefulness on a bike anyway, and a spare patch kit. haha. Ok, it doesn't have a big enough open end wrench to disassemble my head assembly either, but nor should it. It's for on-the-go repairs.
Maybe it's just extra weight, but I like having it along.