Connect your midi instrument to your computer and jam with musicians around the world. I built a similar technology almost 20 years ago so a friend and I could jam together through a 1200 baud modem connection. Unfortunately, there was enough delay that it wasn't much more than an interesting curiosity.

This site charges to jam with others, but as the mechanics are very simple, if it becomes popular, I wouldn't be surprised to see open-source versions pop up quickly.

Link