I've been using D16's Phoscyon for a couple of years now, and added Nepheton to my production environment as well. These 303 and 808 virtual instrument plug-ins are the best out there. They are the best because they don't stop at emulation; they take the original analog instrument further, adding functionality to suit the digital realm. Great sound meets ease of use! That being said about the instrument plug-ins, it was no big surprise that their effect plug-ins were also quite amazing.
I recently had the opportunity to use Decimort during the mastering process on a track where noise and gradual destruction was the theme. Decimort is a truly unique plug-in. In addition to bit crushing, the ability to fine tune to a specific frequency, select a filter type and sweep the cutoffs really set it apart. Use it for all out bit crushing to destroy the audio, or to subtly color and warm a track. All parameters are fully automatable via DAW. An extremely versatile effect.