I have assembled my fair share of IKEA and Target furniture over the years and it seems like everything comes with its own Allen wrench. Often, furniture needs to be tightened or even completely unassembled so it may be prudent to hang on to the Allen wrenches.
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