Tiffany Brown has pointed out a video from this year's WordCamp WordPress event - a presentation from Will Norris, "How NOT to Build a WordPress Plugin".
An amazing video from WordCamp Portland 2009 with tips on how to build a better WordPress plug-in. Lots of nuggets about functions and constants that aren't covered in the codex.
You can check out the video here (it comes in at about 45 minutes).