Multi-tool.

I carry a swiss army knife. Not because I'm an insane stabby maniac, but because I find it useful for doing stuff with. It really is very handy. Today I bought myself a can of food from coles, and only when I got back to the art room with my poor man's lunch, did i realize it had no opening key. If I hadn't had my multi-tool, I would have looked like an idiot.

It's a great thing! :D Leather-man, Gerber Multi-Pliers or Swiss Army Knife, it doesn't really matter, I recommend you get one.