There's an incredible everything-but-the-kitchen-sink calculator that runs on PocketPC, as well as on various flavors of Windows (have you ever seen a calculator with the periodic table and a summary of the properties of each element built-in? I hadn't.).
It's got an incredible array of conversions and constants, it does scientific and financial calculations, supports arbitrary bases (not just binary, octal, decimal, and hex), does algebraic and RPN entry, etc., etc. And it's FREE!
www.calculator.org. Really cool stuff.
(One note -- in the Pocket PC install, the HTML help directory is omitted. It's in the ZIP file, so it's easily copied from the PC to the Axim, if you have space for it. You'll need it to find all the features.)