I've taken to having three 3/8 drives with 10mm sockets and extensions on hand at all times: one for under the hood, one for inside and one for under the car.
Things that go missing:
pick set (since they are really handy for lots of things)
medium sized prybar
drill bits (usually in the house instead of the garage)
small, flexible tape measure that I went to the fabric store to get (no idea where that went)

