I came across this Tricorder app in the market that is just awesome! It uses the sensors in the Droid (might work on other android phones?) and measures gravitational & magnetic fields, geo information, decibels, Solar information, and radio signals. It's just a fun app to play with (especially the decibels!)
Tricorder v4.2 Application for Android | Tools
Tricorder v4.2 Application for Android | Tools