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It employs technologies (like AJAX) to make it act more like a desktop application, which dramatically improves usability.
Many users enjoy the benefits of a native DHTML application, as it does not consume as many system resources as a Java applet (such as ICQ2Go or Yahoo Web Messenger).
Some users are also experimenting with setting meebo as their Active Desktop page to use meebo like a native Windows application for their instant-messaging purposes.
So Go Meebo!