Google began inviting volunteers to a public preview test of their new Wave web-based collaborative email and document communications platform yesterday, which enables users to "communicate and work together in real time." Initial reviews this past May seemed positive.

Features include real-time collaboration, (including concurrent editing and control of each "wave" thread,) natural language tools and a variety of APIs which users can use to embed content in other sites.

Google Wave Developer Preview at Google I/O 2009 (80min video -- an abridged 10 minute version is here. They mention that the term "Wave" comes from Firefly.

Wave login page is here.

Google Wave Federation Protocol is an open source project for developers.

The WSJ reports that invites are now for sale on EBay. Something similar happened when Google launched GMail in 2004. And of course, malware peddlers have turned out in force, so be careful what you search for.