I saw the first demo of Google Wave today at Google I/O. It's a collaboration platform that combines the best characteristics of email, bulletin boards, instant messaging. It also has some of the characteristics of a social network site. It started out as the Next Big Thing from the Rasmussen brothers, the inventors of Google Maps. It's been a couple of years in development and it's not yet publicly released. It consists of the client program, a set of APIs, and a protocol.
The user interface is really well designed, but its real strength is the real time updates, taking the IM paradigm to the next level, i.e. collaboration. This is a really great feature because it means you can collaborate on the same "wave" (conversation) and see everyone's updates in real time.
While it didn't start that way, it seems now to be aimed at stealing some of Facebook's thunder. Google wants to do an end run around Facebook by open sourcing the source code, but even more importantly, by allowing anyone to host a Wave service. This will enable enterprises to host their own private instance; but the really powerful thing is that instances can be federated, i.e. they can communicate with each other, but they've taken care to make sure that privacy is maintained.
The APIs and protocol will be available at the same time as the product itself.