Mercuriall Tubes Creamer
Mercuriall are known for their solid guitar pedal effects, and the Tubes Creamer is one of their free options.
This particular VST plugin designed to model the TS808 guitar pedal, and it sounds really crunchy, which can be great for getting rough guitar-like tones out of anything.
They also have some other great free pedals on their site, which you should check out.