Learn to play guitar with this beautifully designed site. Work through the lessons using your own guitar along with your computer's microphone so the site can interact with you as you play in real time. The site will even help you tune it. If you haven't got a guitar to hand, you can use the on-screen virtual guitar. The course is designed for beginners, but lessons progress quickly. The site seems to work best on Google Chrome.
http://ictmagic.wikispaces.com/Music%2C+Sound+%26+Podcasts
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