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
"Two child-friendly music Apple apps. A sound sampler which record sounds and play them back to make interesting 'music', the other is drum machine. Supply your own earplugs!"