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Perhaps most interesting: a complete revision of its Radar page, a novel way to find hot content that shirks the obvious "Digg like" interface for a more inventive grid layout.
The changes have been universally welcomed by Tumblr's self-reported 625,000 account holders, and we see them as a welcome upgrade to an already enjoyable service.
Mashable uses Tumblr, too, of course: you're welcome to follow our Tumblelog at mashable.tumblr.com
See also: Tumblr: What's the Best Tumblelog?