Oh man, I had completely forgotten about that. I was so excited about the project when it was announced (I was at pretty much the perfect age to get into following free software projects), and used it on Windows for years. It was actually a pretty decent alternative to using iTunes for a while, which was pretty slow and flaky on Windows, at least at the time.
Pretty sure I contributed an add-on or some code, or something. Also followed Nightingale for a while, mentioned by a sibling comment.
I created a few addons and worked on the application itself, continuing with the fork called Nightingale after POTI discontinued Songbird.
One of the addons I created was TagNinja [0] which let you automatically tag, rename and download cover art for entire albums. It tried to match your naming to an online database and used the best match to automatically update your local files. Worked pretty well, but sadly there are no screenshots left :(
I later created an application on XULRunner that let you search for music which was then searched on various online music sites and played immediately. Kind of like Spotify.
The development experience was awesome. Thanks to XULRunner, there was an integrated update mechanism (there was a program to create a binary diff between two versions of the application, and you needed to place an XML file on the server specifying the path to the update file. This was automatically detected by the application and it would update in the same way Firefox does). You could also use Firefox' download manager. Everything was just ready to use and had a really nice UI. I'm definitely missing it :)
Pretty sure I contributed an add-on or some code, or something. Also followed Nightingale for a while, mentioned by a sibling comment.