Having grown sick of the undocumented, dubious math often found on internet facing BMR calculators, I decided to build my own.
This calculator will determine your basil metabolic rate using both the Harris-Benedict and MD Mifflin ST St Jeor equations, as described on wikipedia.
Anyway, here is the calculator. It's not pretty, as it is mostly for my own use. If anyone would like the code, you're welcome to it. I'll happily distribute it under GPL.
Update: I also built a calculator for Katch-McArdle, which estimates BMR based on lean body mass. Here it is.