ATKTransientShaper is a mono transient shaper with all the parameters usually hidden from the user.
Swell is a free mono auto swell vst plugin effect. Auto swell lowers the gain after the input signal goes over the threshold.
ATKSideChainExpander is a free VST plugin. This stereo expander can work on two channels, left/right or middle/side, possibly in linked mode (only one set of parameters), and can be set up to mix the input signal with the compressed signal (serial/parallel compression).