These guys always mystified me in school and they just keep getting better and better. Here are a couple of brilliant tools created by folks I know from my SAU days.
(click on the images for links to the tools)
by Zach Gray and others
Utility for “mirroring” poses (so you can mirror your walk cycle poses, for example) that works on any character, and on any user defined plane (not just across a default world axis).