I am using free software from SlimDevices (SlimServer and SoftSqueeze), several of the computers on my network, and the Nokia 770 to recreate a Sonos-like experience in my house.

I have SlimServer running on my home network file server (where all my music files are stored). I have SoftSqueeze running on my Media Center computer (connected to my home theater and TV in the family room), a general use laptop in a common area of the house (with wireless USB stereo speakers), and a computer in my office/study.

Using the SlimServer web interface, accessed from the 770, I can control the playback of my .mp3s and internet radio at all 3 locations. The software was free, the computers were already in place, so all it cost me was the $350 for the 770 (this was one of the primary reasons for purchasing the 770) and $50 for the wireless USB speakers for the one computer. The Sonos system is very nice, but for less money, I have essentially the same capabilities, with the added functionality of the 770.