One of my dreams which came true due to work for bandwagon (1) was inventing and writing an embeddable music player. The one embedded in this post plays tunes recorded by one of the best bands on earth - Mouse on Mars (2).
Technology used to achieve the effect wasn't very complicated. It's a flash player which loads a list of streams to play from API.
Standard set for a widget.
To embed the player on the page one have to just pick the playlist from the site (button get jukebox) on the splash or just visit the embed site. You may pick the "your google page" option if you like. Happy listening.
I must say that writing widgets such as that one is a really good fun. You have to play with so many toys (in this case Actionscript, PHP, MySQL, XML, and player authentication). And the best revenue is when you see that it works and people are actually using it (Coda, quite great, Louise, MrGod, and many more).
Somebody may say "MySpace people ... ", but you know they are who I was designing for.
Take care and enjoy!