The wonders of modern technology allow you to jam online with your friends or even total strangers. Try these sites:
Ejamming - jam and record your music online. I'm trying this one out. Will let you know.
Indaba - listen to music or works iand give your feedback; join in the music-making sessions or start your own.
Jamnow - a musical collaboration site where you can jam with other musicians. Jam sessions can be broadcast live.