Musicbox was the my first project at Lullabot that I was on-board from start to finish. My role was primarily theming for this project, though as development progressed I worked heavily with the Brightcove media player, actually correcting a few bugs for them in their brand new player API.
This site also laid the groundwork for creating the fivestar module which we used in the Sony Multisite project. We found a lot of solutions for rating content in this project, although some of them weren't implemented until we started the site for Chris Daughtry.
This was also the project that got me started with imagecache for which I'm now a maintainer. If you're building an image gallery, I highly recommend imagefield/imagecache/views combo over the image module. The initial setup is a little rough, but is magnitudes more flexible.