Google Wave was announced yesterday. If you haven't yet, check it out. It provides a hosted messaging service that, with a few simple features, supports all of: email, instant messaging, and collaborative document editing. It also has an open extension mechanism, so it can be used as a substrate for specialized applications, like blogging, photo album sharing, and issue tracking.
I haven't perused all the web information myself, but I hear that the Tech Crunch article is quite good. Also, if you have some time, the demo is mind-blowing.
Features and vision aside, I must say they made an excellent choice in implementation technology.