With major hardware vendors shipping systems preinstalled with Ubuntu, and the extremely hassle free Ubuntu installer, more users have, if not started mainstream usage, atleast experimenting with this flavor of Linux. Here are some open source (read: free) stuff that will get you started with Ubuntu 7.04.
1. Amarok
Amarok is a unix/linux music player with a nice interface, playlist manager, 10 band equalizer and also automatically fetches your album cover art.
sudo apt-get install amarok
2. VLC
With your music needs sorted out with the above, now to your videos. You needn't look any further than VLC, by far the best cross platform media player. You could also install the VLC plugins and use it as a streaming server.
sudo apt-get install vlc vlc-plugin-*