The release notes are here (and still being updated): - http://docs.moodle.org/en/Moodle_1.9.9_release_notes - http://docs.moodle.org/en/Moodle_1.8.13_release_notes
Moodle 2.0 Preview 1 is now available!
-http://docs.moodle.org/en/Moodle_2.0_Preview_1_release_notes
