Plumbing Tools - Faucet Valve Seat Wrench
Don't you hate having to buy a tool that you'll only use once, maybe twice in your lifetime? Me too. So when I had to buy this $7 thingy to fix my shower faucets, I figured I'd leave it at i3 for others to borrow.
This is one of the few (only?) tools that members are welcome to take home.