In order to celebrate the launch of this year's Google I/O conference (ongoing now) Google has opened their Wave chat platform to the public. No invitation is needed now. Visit this site and sign up now! If you represent a school, business, or other large group, you can enable Wave for all your users now. Free.
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The Wave blog has more details, including information on how Wave has been used so far by high profile organizations. Many changes have been made since Wave first launched. Email notifications have been added, you can now remove participants, and permission management options have been added.