I have a pretty cool rat clone from Freakshow Amplification. Here's part of the description:
"I added a few clipping options, position #1 is Asym clipping (smoothest and most compressed), position #2 is no diodes mode, this is very loud and crunchy...great for DOOM!, position #3 is LED diode clipping mode....this is a less compressed AND FIZZIER version of mode #1."
Def love sending my bass through it as @Spaceboy mentioned; it sounds great.