Bondhus "L" shaped Allen Wrench
The ball-head lets you go in at a slight angle, a real blessing when mounting racks or bottle cages. Actually, "a real blessing" doesn't go far enough. Mounting a single bottle cage without one of these will drive you bananas, and after that, you'll kick the cat.
Made in the USA, top quality, and when you need one, nothing else even comes close. If you value your sanity, you'll own one of these.
Made from Protanium steel for long life.
3mm x 100mm - For little bolts, some brakes and lights and other teeny things.
4mm x 112mm - Bottle cage and rack bolts.
5mm x 121mm - Most popular size, for stems, handlebars, brake levers, shifters, racks, all over the bike.
6mm x 140mm - For stems and the seatpost.
8mm x 163mm - For allen style crank bolts.