Imagine all (and I mean all) the web tools we now use in education. Blogs, wiki, googledocs, photosharing, IM, twitter, etherpad, maps, games, polls, forums and video etc.
Now imagine them all integrated into one web application… cross platform and cross device. Where a tweet shows up instantaneously as a blog entry, a blog comment appears as a tweet, photos appear and disappear on different blogs as they are uploaded or deleted. Collaborative work on documents, images and maps is live and instant. Intelligent spellchecking (using context, symantics and syntax) fixes most errors on the fly.
Google wave is all this and more. Have a look at the developer preview video (1hr 20mins) and see if it moves you to tears like it did me.
... and, oh yeah, it's opensource.