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So, Google released their instant messenger some hours ago. Apparently you do need a Gmail account to use Google Talk but for now I can say that it is an incredible lightweight (900k only!) with phone and IM Power behind a clean interface. Read the reviews at DownloadSquad or try it at your own:

Google Talk

Meebo is a smart online instant messenger tool made in AJAX. You now can chat to your friends whereever you get an terminal without installing additional software. All web based, so here you go:

... http://www.meebo.com/