I bought these a couple months ago for my PC. Replaced a set twice as big and with a big separate sub-woofer. Really cleared up my desk. (Amazon.com product link shortened)
They are USB powered, have good sound. Heavy metal cases, but small. About 5x5. 2 1/2 wide. Sub-woofers have good bass.
And I bought these a couple weeks ago, for my wife's PC. http://www.newegg.com/Product/Product.aspx?Item=N82E16836121013
They are powered by AC, and also have good sound. Plastic cases, light, and about half the size of the other ones. No sub-woofer.
I can recommend both of the above. The first one is better to my ears because of the sub-woofers. You can read the reviews on both.