SEGX2 is a free smooth dual band expander gate audio plugin. SEGX2 operates always with soft knee which starts at the threshold point. Signal above the threshold passes thru unaltered.
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).