We're particularly fond of the Blue Microphone Snowball series. They are really user friendly - plug and play, no additional drivers required, and has a easy set up stand. A separate microphone is recommended for any class where sound is very important - music or voice classes.
You can get them from amazon.com: http://www.amazon.com/Blue-Microphones-Snowball-Microphone-Aluminum/dp/B002OO333Q/ref=sr_1_2?s=electronics&ie=UTF8&qid=1328908273&sr=1-2
The microphone is compatible with Windows 7, Windows Vista, Windows XP (Home and Professional), and Mac OS X (10.4.11 or higher), and requires a minimum of 64 MB of RAM
As we get new ones in the office, we'll post more recommendations of our favorites!