The ATR2100-USB from Audio-Technica is a fabulous microphone as either a main mic for a casual podcaster or as a mic for use by guests or in the field. It includes an XLR connector for studio use and a USB connection for stand-alone use. There is even a headphone jack to hear the return audio as you would want with Skype. Best of all, the microphone is available for less than $50US. It's not a Heil PR40, but it's a lot more useful than a Shure SM58.
UPDATE: April 14, 2012. Leo Laporte has just discovered the awesomeness of the ATR2100-USB microphone. At one point during his show, he did an A-B comparison with the fabulous Heil PR40. The difference was not as dramatic as you would think.