Apple's iTunes will share music files with other computers running iTunes on a local network, so that those other computers can listen to your digital music collection like a radio station. (They don't get copies of the songs.)
If you're a Mac iTunes user and would like to share your music with people who don't run iTunes—whether they're on a Mac, Windows machine, Linux box, or whatever—or who are outside your local network, TunesAtWork runs a Java-based mini–web server that lets anyone listen to your MP3 files if you give them the appropriate web address (and allow the streams through any data firewall you use, if you want people outside your local network to have access).
And it's free. Cool!