Alexis Isaac has released the beta of a very interesting project called FlashMidi: it’s an API that allows you to send MIDI events from Flash to any of the 16 MIDI ports in your system. This means for example that you can play all the instuments that are built into your soundcard. In order for this to work you have to install a tiny .exe plugin he has written - so currently it runs on Windwows only. The API is very well documented and there are also some examples available.

As future additions he plans to implement MIDI-input - which I think is very interesting as you could use this also for all kinds of experimental interfaces, which do not necessarily have to be music-related.