A very useful expression to automatically fade in and out based on the layers length.

Ever wanted to fly around a "CC Sphered" Layer? Linking CC Sphere to a Light and Camera is the way to do it.

If you would like to reference one expression from different layers to change things only in one place.

This script creates and animates a chain of 3D falling dominoes.

A great time-saving preset to "duck"(fade in/out) a music layer based on a voice over.
"Ducking - Audio Terms Explained - Process of automatic compression, eg when the announcer's voice signal causes the level of music to be attenuated."(from lp2cd.com)

If you want to use the speed/velocity of a layer's movement to drive an effect ...

Film Burn is an Animation Preset that automatically creates the overexposed "roll-out" effect, where the tail (and the head if you want) of a clip flickers into overexposure. By Stu Maschwitz

This script can be used to quickly turn layered artwork into a 3D multiplane scene.

Create a futuristic sniper scope using After Effects expressions and null object animation.

A practical application of a layer space transform.
Not very new, but still very useful.
by Dan Ebberts's resource for After Effects scripting and expressions