Here’s a few USB powered microphones and kits made for podcasting or the like. When USB microphones first came into this world they got a bad rep among professionals. Since then it seems like things have change, at least a bit. I have no idea about how good these particular microphones are though, so you’ll have to try them for yourself.
This is a beauty…
Alesis USB podcasting mic kit
I remember reading good things about Røde USB mics.
This is a USB bundle from the controversial company Behringer. It seems like they have a Firewire bundle as well.
Behringer Podcast Studio