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The One I Use: Samson Go Mic
The microphone I use to record my instructional videos and podcasts is the Samson Go Mic, a very small, inexpensive microphone that I plug directly into the USB port on my laptop. I originally bought it because it was the highest recommended USB mic at a very low price. You can literally fold it up and stick it in your pocket:
Samson Go Mic Portable USB Condenser Microphone (About $40)
Other Alternatives
Here are some other more expensive but highly rated microphones. Both are also USB plug-in microphones, ideal for the home user.
Blue Snowball iCE Condenser Microphone (about $50)
Blue Microphones Yeti USB Microphone (About $130)