Moodle's Open Badges plugin. Still in alpha and planned for a Moodle 2.5 release (I think).
Have managed to have a play with it on a friends install and it is AWESOME.
See the other Scoopit entry for details on the documentation.
Via gideonwilliams, Nadav Kavalerchik, Gisele Brugger, michel verstrepen, Rui Guimarães Lima