Facebook Open Platform. Facebook have open-sourced (under a modified MPL, does it still fit the OSI definition?) the code for the Facebook Platform, including their implementations of FBML, FQL and FBJS. This is no small release; the tarball weighs in at 40MB and includes libfbml, which depends on Firefox 2.0.0.4 for its HTML parser!
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The license that Facebook is using is indeed certified OSS. (This was the first thing I checked when this came out.)
http://www.opensource.org/licenses/cpal_1.0