It would be virtually impossible to cover all of their capabilities. It's really limited only by your ingenuity. That said, I've done a little bit:
Blackmagic Fusion: Expressions
Soft-Skinned 3D geometry
Linking 3D Geometry to Particles
Pin Light Blend Mode
That's everything I can recall writing. At least, everything that's available to the public. I've got some stuff on the Muse internal wiki that I should repeat on my blog eventually.
I recall seeing an interesting video about pCustom for motion graphics. There are also some interesting applications of custom tools in several macros available at We Suck Less. For instance, SecondMan_Yankee is cool.
edit: Made nicer links.