Three desks out of these 20 (the ones belonging to Jeff Croft, Dan Mall, and Veerle Pieters) are seen with Blue Microphones Yeti USB Microphone.

Jeff Croft deskJeff Croft's desk with Blue Yeti.

Looks a good shout. If you have a mic recommendation it'd be great to know.

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Update:
I went for the Yeti, and it's obvious the sound is better than through the iMac's standard mic. Definitely less echo from the hard floor and relatively sparse walls in my workspace.