The Music Nomad Octopus is a great little tool that I've kept in my gig bag ever since I got one.
It's got 5 different sized sockets that you're likely to find on a guitar: 10mm 12mm 14mm 1/2" and 7/16". There's an extra 1/2" socket that's good for getting at those recessed output jacks on telecasters, and a phillips and flathead screwdriver.
Mine has saved the day at a number of gigs already and I've only had it for a month.