The user ‘Beaver’ posted 5 Expressions that will change your life on the Mograph forums.
Colin Braley provides a tutorial and example project on his website that show how to use expressions to make one layer repel others in a natural-seeming manner.
The AE Enhancers forum provides many examples and much useful information about expressions, as well as scripts and animation presets. In this post on the AE Enhancers forum, Paul Tuersley provides a tutorial and example project that show how to use expressions to animate several layers in a swarm.
Rick Gerard provides an example on his website that demonstrates rolling a square object along a floor so that the sides stay in contact with the floor plane.
Carl Larsen provides a video tutorial on the Creative COW website that demonstrates how to use expressions and parenting to relate the rotation of a set of wheels to the horizontal movement of a vehicle.
Chris Zwar provides an example project on his website for automatically arranging still images or videos into a grid (like a video wall). You can easily adjust position and spacing with sliders that are connected to a system of expressions. There are three compositions in the project—one for stills, one for videos, and one to create an auto-storyboard in which a video is sampled at user-defined intervals and aligned into a grid.
JJ Gifford’s website provides several example projects that demonstrate how to use expressions.
Maltaannon (Jerzy Drozda, Jr.) provides a video tutorial on his website that shows how to use expressions to create a volume meter using the results of the Convert Audio To Keyframes command.