I just spent the larger part of my weekend extending my Moodle 1.9.14's HTMLArea Toolbar. Many thanks to the prolific and brilliant Nadav Kavalerchik. See the moodle.org plugin page.
Hadav's work is amazing!. He has increased the pedagogical value of Moodle right from the Editor itself.
For me the most useful plugins that has has added to the Moodle editor functionality are:
3. VideoRecorder (FANTASTIC. Thanks for Kevin Brake (and Vimas Technology)!)