Clamps at good prices. I don't even know how many I own but I've ran out and gone to the store a couple times in the past year. Most of the time they just set there not being used, but when you need them, you need them and they are so cheap that I can easily justify another 2, 5, 10, whatever here and there as they hit steep discounts.
I'm also rather fond of socket extensions of various sizes and configurations. There always seems to be that one oddball bolt that takes some odd size/configuration of extension and having the right size/type of extension on hand makes a difficult job into a painless job really quickly. I have regular ones, locking, quick detatch, wobble heads, adapters, etc all in lots of sizes for each drive type and yet any time I see something in a configuration I don't have I feel I've got to pick it up.