Since audio output is not currently working on my Windows XP install on my Mac mini, I have been shopping around for an optical or USB-based audio solution. Google doesn’t really yield very much, as I guess that USB-based audio is fairly unnecessary for most people, especially since most computers have perfectly good audio already built-in.
Digital audio (optical or USB) is useful when you want to play or record as near to the original signal as possible, since the external hardware can be isolated from the various hums and clicks of the PC circuits.
The front runner so far is the Griffin iMic which I have suggested as a great solution for the Apple iBooks, which do not have a microphone or line input.
The iMic is a great example of the ‘Apple style’, being simple to use, with no fuss or configuration required, beyond selecting it in the sound preferences of the OS X configuration page.
In contrast, Google also shows these examples of the contrasting, ‘put it in an ugly box and get it out the door’ philosophy. 
I’m boycotting anything ugly. Hopefully the manufacturers will start to notice that good design sells.
Turtle Beach makes a good one. Here is one link:
http://www.thinkgeek.com/computing/avcards/6e4f/