My vote is for the Sonica of Sonica Theatre. I've got the original Sonica, actually bought it at MacWorld NYC a couple years ago right after they came out.
With Panther, it works like a dream through DVD Player and also through VLC to do multichannel. It does Dolby Digital and DTS and the fact that it's USB 1.1 is not a problem at all. I even use it through a hub.
There was just a driver update in the past couple days, so I'm going to check that out when I get home and see if it changes at all.
The Sonica is only about $50 online, maybe around $90 for the Sonica Theatre. The difference is that the Sonica Theatre has analog outputs for up to 7.1 and optical out, the Sonica has stereo analog and optical out only. Both are bus powered and very compact.
cpjakes