Satya Meka provides a good overview of the different ways to develop effect plug-ins for After Effects:
“Intro to After Effects plug-in development for beginners”
One of the ways to create effect plug-ins for After Effects is using Pixel Bender. Creating plug-ins written with Pixel Bender tends to be easier than creating plug-ins written using the C/C++ SDK. Also, the ability to share extensions between Photoshop, Flash, and After Effects is a pretty big advantage.
Satya provides an in-depth introduction to Pixel Bender, too:
“Tutorial: Intro to Pixel Bender for Non-Programmers”
For more information about Pixel Bender, including tutorials, reference documentation, and examples, see the “Pixel Bender Technology Center”.
Satya mentions the terrific fractal effects written by Tom Beddard using Pixel Bender. The image at the top of this post is from one of Tom’s Pixel Bender effects, which he calls “Escher’s Droste effect”.
For more information on installing, developing, and finding plug-ins, see “Plug-ins”.
UPDATE: Rich Young has a good summary following up on Satya’s post.