Im a big fan of the Winamp AVS (Advanced Visualization Studio), basically a fully programmable audio visualizer. Its not actually in development any more but would be so cool to see it ported to linux and use it on my pi. The source is available so if anyones interested in this project please take a look. If theres one thing xbmc needs, its this plugin.
Heres some awesome examples: http://acko.net/blog/avs/