At the first GHC Hackathon (Portland 2006), the two Simons gave a number of informal presentations about the internals of GHC. Since they mostly talked around the wiki material in the Commentary, I have tried to attach the videos to the relevant pages. A full list also appears below.
The quality of the audio or (particularly) video may not be great. They are intended mostly as a guide to the wiki commentary, so try to make sure you can see the wiki pages side by side with the video, because it may be difficult to make out the visual details otherwise.
Intro to the Hackathon, purpose of the event (6'47")