Many of the wrenches for assemble-it-yourself furniture are interchangeable but some are size specific. I got a teeny tiny Allen wrench with the bathtub faucet handle I installed last year that I'll definitely need again in the future. So I keep all of the wrenches in a labelled envelope in our junk/tool drawer. To keep track of the Allen wrenches I label them with painters tape. Of course, I could buy a set of Allen wrenches but why do that when I keep getting them for free?
Images: Jason Loper