While working on an electronics design project, I found myself doing a decent amount of basic math on quantities with very unlike magnitudes. I just wanted to be able to multiply microfarads by kiloohoms without having to remember the various exponents in my head.
I looked around for a while, but I couldn't find a calculator that did exactly what I wanted, so I wrote metcalc. It understands most of the standard metric units and lets you do math with them directly. Some examples:
To get a feel for how it works, type 1M/1 into the calculation box and keep adding zeros to the end. It will step through all of the units that it recognizes.
It only supports the four basic math operations (+, -, *, /) and parenthetical expressions. If you're a coder and feel like contributing, check out the repository on github. If you need a refresher on metric suffixes, I pulled my metric units list from wikipedia. If you need a referesher on me, check out my my blog.