This is an amazing site which allow up to 4 people to collaboarate and play virtual instruments over the web in real time. Play a variety of guitars, drums and drum machines and keyboards. There is no sign up needed and you just invite other 'musicians' by sharing the link and begin jamming. Set to 'pro' mode to play using your computer keyboard. Requires Google Chrome.
http://ictmagic.wikispaces.com/Music%2C+Sound+%26+Podcasts
"A wonderfully designed site with lessons and tools to create digital music and teach music theory. Lessons start at a very basic level and build to advanced compositions."