Pluggo is a classic sound design tool, with plugins ranging from utterly useful to utterly bizarre and destructive. Now, Cycling `74 is giving away 12 free Pluggos, calling it “Pluggo Jr.” The selection actually looks quite nice; see JDG’s story on em411. Windows-only for now, but Mac promised soon.
Of course, the best way to go is a tool like Max/MSP, which was used to create these plugins. You know, when you want to create your own plugins that completely destroy any input fed into them (sorry, maybe I’m projecting).