Mutronics MIDI Mutator
Most likely the best stand-alone rack mount stereo (two channel) filter, modulation and envelope follower unit. It has two great resonant filters with cutoff, envelope follower, and an LFO with 4 waveform types that can be linked or inverted. The Mutator is truly a great dance music machine. Run loops, synths, voice or entire mixes through this and filter them in stereo or dual mono. It sounds great!
The two filters are identical and totally independent of each other. The circuitry is analog too. The VCF (filters) have their own envelopes and there is a useful LFO for nice sweeping or pulsing effects. The LFO can also be MIDI synced to your music (it's a little tricky though).
There is a great implementation of MIDI control on the Mutator. Control the filter cutoff from your sequencer by hitting notes on the keyboard. That's it, the notes on your keyboard and the pitch-bend correspond to the different degrees of cutoff. There is no patch memory, but that's ok. The Mutator is a machine that's meant to be grabbed and played with live while either going to tape or live on stage. Filtering music for an effect has become very common these days and the Mutator is among the best machines for achieving this effect. It is THE SOUND used by Daft Punk and has also been used by Fatboy Slim, Future Sound of London, Depeche Mode, and Radiohead.
Demos & Media
Images from yousenditworks eBay Store.