Audio Technica 2005 USB Podcasting Microphone

http://www.schoolofpodcasting.com/youtube

This is a great DYNAMIC USB microphone that can operate as an XLR microphone as well and it sounds great. It's around $80 on amazon at

http://www.schoolofpodcasting.com/2005 and it's little borhter the Audio technica 21000 goes for around $49 at
http://www.schoolofpodcasting.com/2100

Both are great mics that allow you to grow.