It's cheap, it's compact, and removing a single key doesn't require you to tear your fingernails to shreds trying to pry open a keyring. And because Lego is produced to such exacting dimensions, there's little to no chance this thing is going to come apart until you want it to. As a bonus it's even easier to build a spot to hang the keychain when you get home—just stick a Lego baseplate to the wall near your front door. Done. More here.