Once I saw a program that allowed you to create your own plugins, I think. But most of the pictures of outputs from example plugins seemed pretty craptacular. So I really don't know if you'd even want to go finding it.
The only thing I know about plugins is that the Alien Skin plugins (as well as most others, I'd guess) are somehow based on mathematical formulas. Insert graphic "A", run weirdo freaky mathematical formula "B", output expand-o-graphic marvel "C".
---Skorpion
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strategery.
dignitude.
lockbox.
