Vista can stream music, videos, and the like between the networked PCs in your house. That's nice if you're at home, but what if you want to enjoy your media collection elsewhere? Simplify Media can stream songs in various formats from your desktop music library (or tunes from any friends you care to invite) to any other PC. There's also an iPhone app (Simplify Music 2.0) for streaming on the run. Both the software and service are free.
Note: This link takes you to the vendor's site, where you can download the latest version of the software.