Camel crusher is a free to use distortion plugin with multiple effects.
There are two distinct types of distortion you can add with this VST/AU.
One is a but warmer sound which ads that analogue feeling to your sound with the tube emulation.
You also have the digital preset which gives you that gritty bits like sound.
Combining the two presets gives you a new range sounds you could shape, with the added compression it really beefs up the sound.
Available for PC and Mac.