From Wikipedia:
Google Wave is a self-described "personal communication and collaboration tool" announced by Google at the Google I/O conference on May 27, 2009.[1][2] It is a web-based service, computing platform, and communications protocol designed to merge e-mail, instant messaging, wikis, and social networking.[3] It has a strong collaborative and real-time[4] focus supported by extensions that can provide, for example, spelling/grammar checking, automated translation among 40 languages,[2] and numerous other extensions.[4] Initially released only to developers, a "preview release" of Google Wave was extended to 100,000 users in September 2009, each allowed to invite twenty to thirty additional users. On the 29th of November 2009, Google accepted most requests submitted soon after the extended release of the technical preview in September 2009; these users have around 25 invitations to give.