From FabFilter Software Instruments:
FabFilter One is a cut-off filter with a new design approach, which results in fattest and smoothest self-oscillating digital filter. It has been tuned carefully to achieve the character and warmth of a classic analog synthesizer. FabFilter One's filter sounds raw but never cheap or 'digital', even at full resonance.
The filter is fed by an equally high-quality solid-sounding oscillator, developed from scratch to be completely aliasing-free. It features sawtooth, triangle, and square wave forms with pulse width modulation. The oscillator and the filter can be modulated by the LFO and the EG. The LFO offers triangle and square wave forms, with adjustable wave balance so any wave form between sawtooth and triangle can be selected. The tempo can be synchronized to the host. The EG has the usual ADSR knobs, but with a perfectly fine-tuned feel. Also, it has an additional Hold setting that extends the time keys are held down.