ReaComp

This was notably the first free compressor with a feedback ability.

It doesn't have loads of character, it's meant to be more clean and basic. Like all Cockos software it's efficient and stable. I remember loading up projects with tons of instances with little CPU draw. It is still useful as a basic channel compressor.

ReaComp offer a lot of parameters, maybe more than all others compressors. Internal & external sidechain, wet/dry mix, peak/rms detector with variable rms time, lookahead attack... All this with light Cpu weight.
It have also level meters and indicators.

...Try the power of pumping-effect on a track with ReaComp triggered by a drumkick!

You can find this Vst plugin alone, or in bundle with Cockos Reaper audio recording software and Midi sequencer, the affordablest (and newest) instrument for music creation.