Open Source: Visit the Google Code page for full source and feel free to submit improvements!
Here's how it works:
android-http-mediabutton runs a web server on your phone.
You connect to it from a browser.
You click the buttons (play/pause, next, previous, stop).
android-http-mediabutton simulates these button presses.
Now you can plug your phone into your stereo, fire up any media player, and control it from a web browser.
Bugs? Don't complain, contribute!