I remember when I got my Topeak Alien and thought it was the greatest thing ever. It was so cool, it split into two parts. I lost half of it.... Well, maybe I lost it, but whenever I see one half, I've forgotten if it's the half that was lost.

I later got a CB 17. It's so much slimmer and doesn't require you to take it apart. I've adjusted derailleurs, aligned brakes, repaired chains, and trued wheels with it. All the other flat designs are cool, too. Park Tool has a truly tiny mini I-beam that I carry a lot, and somewhere I've got a Specialized that has a couple shims for spreading disc brake pads.