Mr_elenganza has a great question over on the WoW LJ-- what headset do you use for voicechatting in WoW?
Me, I use two different audio inputs. On my Mac Powerbook, the built-in microphone actually works really well (and I hear from others who've used the new Macbooks that those work even better), so whenever I'm raiding with Teamspeak on, I'll have it on there, and just use that mic to talk.
But when I need something a little clearer, I use the cheaper Logitech headset-- there are more expensive gaming headsets out there, but the headset is really just for voice (I've also got a nice set of 5.1 speakers on my PC), so it works just fine for my purposes. To tell the truth, if I went for another headset, I'd probably try and get something Bluetooth (the fewer wires running around, the better), but I haven't shopped around too much for that-- anyone want to recommend one?
At this point, headsets are so cheap and so commonly used that "I don't have a mic" isn't really a valid excuse for not voicechatting any more (of course, it's always a good excuse if there are other reasons you don't want to speak). A nice USB headset will help you hear what's going on, and will make the experience better for people playing with you, too.