Back N Forth is a free sine wave generator preset for the position, scale, and rotation in every axis of your objects inside of After Effects...
What this means is that with this preset, you can automatically make your layers move from side to side (or up and down) in a motion that is always equidistant from the center on both ends! This is different from using something like “Wiggle” in that wiggle is more or less random motion, whereas sin waves are constant and fluid.
Author Neal Barenblat has another preset as well, Flicker On.
Back N Forth joins other recent expression-based presets as animation aids, like Gravity preset from Jesse Toula and Better Bouncing by Ian Haigh. See foundation basics and much more from JJ Gifford (plus harmonic motion) and Dan Ebberts.
Not disimilar is Ease and Wizz, a script to generate alternative keyframe interpolations for After Effects (especially to the sine-like Easy Ease). Looking at pictures by Chris Meyer is the fastest way to see what behaviors you get with this script. Author Ian Haigh has a nice demo on AE Scripts; here's a preview:
Continuing with tangents off sine animation are some tutorials for creating animation of sines:
- Creating sine animation with the Wave Warp effect by Aharon Rabinowitz
- Generate a Dynamic Morphing Background Design with Sinedots by Stefan Surmabojov at AEtuts+ (below). Sinedots is available free for CS5 and earlier from Philipp Spöth, aka Frischluft (maker of Lenscare and FreshCurves).