Deep Step-606 is a new free VST plugin that promises "the energy and creativity of a classic synth coupled with a vintage analogue step sequencer".
It's based on SonicXTC's existing Deep 606 synth (it shares presets with both this and Deep Mono) but is monophonic. What's more, its sound is completely controlled by the step sequencer.
You can find out more Deep Step-606 in the video above. You can download it from the Sonic-XTC website.