SKnote has released Grasso as a VST effect for Windows and a VST/AU effect for Mac OS X (RTAS/AAX coming soon).
Grasso is a tube emulation effect with saturation and level-independent control on attack and decay of sounds. It is neither a compressor (no thresholds) nor an envelope shaper (no triggering).
It is available for purchase for $29.99.
Some video examples are available here: