SlimServer
I was always interested in the SliMP3, but it always seemed too expensive for my uses. However, the new Squeezebox by Slim Devices is very interesting. It's still a pretty penny, but its breadth of capabilities may be worth it. Being able to stick it (the wireless Ethernet version) anywhere in the house and stream uncompressed audio to it is quite compelling to me.
I'm going to check out the SlimServer software to see how flexible it is. It looks like it supports non-proprietary MP3 HTTP streaming.