Dan Frakes, of Playlist, has reviewed two USB microphones: the Blue Snowball, and the Samson C01U. Now, if you're podcasting and aren't looking to invest in a mixer and a number of other pieces of audio equipment then a these USB mics are for you. They plug right into your Mac's USB port and get the power they need from your Mac. Dan points out that this convenience does come with a price, namely these mics sound good but not as good as more professional mics.
At $79 the Samson edges out the $159 Snowball in audio quality, but the Snowball just looks so cool (and I should mention that I own a Snowball and I love it).