Anderton
Elffin
Using drum maps to control articulation switches in kontakt... works great!
More stuff I never realized. I'm so glad I started this thread, if only for selfish reasons 
This is a good one which I use all the time. I created drum maps to keyswitch between all the articulations in my EWQL Play libraries. The best part is you get to deal with descriptive articulation names instead of note names and numbers along the left hand side of the PRV.
I use a separate MIDI track for the KS drum map which I put in a folder along with the MIDI notes track. This way I can just select the folder and then open the PRV to see all the relevant MIDI data for that instrument.
Anderton
Here's another feature I think is underrated: MIDI FX. They're so useful for making musically helpful wholesale changes without having to edit on a per-note or per-clip basis (in particular the velocity one).
The quantize one is invaluable when songwriting. i can temporarily clean up a part while the songwriting inspiratino thing is happening, then go back later and do the "real" quantization that's more subtle, uses groove quantizing, etc.
I'll second that. I use the Velocity MIDI FX all the time to randomize velocities in my MIDI sequences as part of my humanizing process.
Kenn
EDIT: BTW, lots of great stuff in this thread. Thanks Craig for starting it! As nobody says, "With great questions comes great responsibility." - I hope you're keeping track of all the gems here to compile into a handy document for the community. Or at least you'll have some fodder for your next book!