The Logitech USB To 3.5mm Jack Audio Adapter allows you to connect an analog headset to your computer via the USB port. Some systems, like older Macs don't have an analog mic-in port, so they'll need a USB adapter in order to connect a microphone.
If anyone has any experience using any of these, share your thoughts in the comments.
UPDATE: Found this one also: the Zoom ZH4N Handy Portable Digital Recorder. It's an audio recorder that also works as an audio USB interface. It has it's own mic, plus a few ports (including XLR ones) for external mics. I'm just not sure if, when used as a USB interface, you can only record from the primary mic, or if it lets you record from any external mic directly to your Mac.