What are you after? Usb powered or bypassing the inbuilt sound card entirely?
If the second option, there are two decent choices:
a) Logitech V20s are the best bang for buck. One USB connection for everything.
b) Logitech Z10s, which pass sound through USB but require AC power. Much beefier sound and some support under OSX for the inbuilt display.
If the first option, then perhaps the Altec Lansing XT1s. Powered by USB but have a line out lead for the headphone port on the iMac.