Even though this virtual slide rule is full of actual win, you can buy a real (if chintzy) slide rule for cheap from ThinkGeek.
I suck at math, and I'll readily admit it. You still need to be okay at math to use a slide rule: namely, you have to keep track of the order of magnitude of your calculations in your head or on paper. It's no secret that I dig old tech, though. Plus, analog calculators are fun and you can't exactly stuff a Babbage difference engine into your pocket.
Related: Upgrade!