Mediatomb is a GPL UPnP media streaming server that will stream all sort of media (MP3, OGG, Flac, images, movies of all kind) to compatible devices. An example of a compatible device would be the Playstation 3 (PS3) with which you can then watch your media streamed via WLAN from your computer on your TV.
Install package for your distribution, it should be called "mediatomb". Look here for information on how to install packages.
Mediatomb is configured in /etc/mediatomb/config.xml. Normally (with Gentoo) the configuration is fine, but you can still tweak some things.
If using Gentoo, you need to edit /etc/conf.d/mediatomb if you are using a network interface different from eth0 (eth1, wifi0, ath0 or something). Remember to restart the service after this and use rc-update to add it to the default runlevel with Gentoo.
TODO here: More tweaks for PS3 compatibility.
You can now browse to the web interface at localhost:49152 and start adding directories containing media data to make them accessable over the network via Mediatomb. Mediatomb supports inotify (and does rescans, if you want, is your system does not support inotify), so it will notice, when the content in the monitored directories changes and reflect the changes almost immediately.