Phlegetron is a multi-distortion effect that works across dual bands, with the bands either separated by a crossover filter or split into separate harmonic "bins". By adjusting the frequency of the split, you can emphasize different harmonic frequencies and carefully tune the effect to target a deliciously nasty zone inside your mix.
It can work well on individual instruments, can be surprisingly useful on string instruments but excels as a sound design tool.
Phlegetron is freely available in VST3 and AU formats for macOS and Windows* and fully open source.
*Linux users can build from source