You can also download the Soundhack program for free, but it only runs on a Mac. If you are a Mac user, as well as an audio geek, you must download this app! The UI is text-based, and manipulating sounds is often a trial-and-error process, but it's well worth it. I created many of the sounds I used in my electronic project using Soundhack!
You might even want to check out their "Spectral Shapers" plugins. They cost money, but you can use it in demo mode for 14 days. I haven't tried it, but it looks like it does some of the things Soundhack does, but with an easier-to-use interface, and the ability to use it within your host DAW! There is an endorsement by Trent Reznor on the website, so that's good enough for me!