Filter Forge is – by quite a margin – the most powerful plug-in we've looked at here. That's because it's a tool for creating plug-ins, both generative texture-creation tools and effect filters.
“Incredibly powerful texture engine.”
“Chief among my pleasures is Filter Forge, which I've been ranting (positively) and raving (voraciously) about ever since it entered my life.”
Resident Enlightenment Manager Linden Lab, makers of virtual world Second Life