FMCalc is a free medical calculator for Windows Phone 7.
It is not only free in regards to price, but free of ads.
Currently, it is the ONLY free medical calculator app on the WP7 OS.
Version 1.1 supports over 40 calculations and criteria.
Unfortunately it is SI units only (not US units), but I will try to convert it if there is interest.
If there is anything you would like added to the application I will try...