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Gabbly, Gabbr, Gather, Mabber, Meebo, Meetro - and now Mobber! You gotta love these Web 2.0 names! Even better, Mobber is another ajax-based chat widget. From the site:
Who's On? Mobber's patent-pending software shows you who's on a particular web page, allowing you to chat with them privately or in a group. We make the web a two-way real-time medium. Got a site, or online profile? Add Mobber to it in under 5 seconds. Go on… mob the web!