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Google Wave was released to developers last year. Then Google made it available to a select few, who then had the ability to invite people to use it with them. Some of those people sold invites via eBay. LOL. Now everybody can access to the tool.
The app is written in the Java programming language. It is supposed to allow you to have conversations and work on a document at the same time. Waves are actually XML documents. You modify the document like you would a Wiki. Google plans to open source the code some day.