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.
“Oh my god, I've got to get into this!”
“Filter Forge is an incredibly powerful new Photoshop Plugin. It's simply amazing. It gives you the power to create your own filters (you must run them from inside Filter Forge though) without any programming knowledge at all.”