This is bordering on thread necromancy, but I noticed these aren't listed...
The Mark Summers Halloween Special. I really loved this one. I remember being all annoyed one year because my parents made me go out to dinner with them. They said we'd be back in time for me to see it, but everything took too long and I missed it.
Nick had an excellent Thanksgiving special that had a few different parts. One of them was about a poor family with nothing more than a can of beans for the big dinner, but the kids got sucked into some kind of dreamworld where a whole thanksgiving dinner was cooked. The next day it was all in the kitchen still.
Who can forget all those stop motion animation specials that were on USA. Rudolph the red nosed reindeer, the island of misfit toys, etc. As I remember, USA network had quite a few good Christmas specials.
Nick had another good special about some toy land, but I can't remember the details.