Todd Kopriva's AE region of interest features a discussion of Dave van Brink, who has a variety of AE projects and scripting nuggets at his omino pixel blog. His free AE filters, the omino after effects suite, are available for both Mac and Windows. He was also one of the original engineers of QuickTime, and his QT_TOOLS for the Mac might be be handy for some (for example, qt_proofsheet renders frames of a QuickTime move to a big picture).
Check out Todd's post David Van Brink on After Effects and the link to the Particle Playground halftone project shown in the movie below:
Update: Harry Frank did a tutorial for a Particular Halftone Effect.