The Humble Audio Quad Operator is an entirely open ended FM synth matrix. This is a four operator FM voice in eurorack form without any preset algorithms, this allows you to create your own algorithms through a aux send style matrix.
The FM voice is created in software as opposed to using a new old stock yamaha chip for instance which allows the unit to be more similar to something like FM 8 or Ableton Operator.
The Quad Operator has 4 variable waveshape operators an audio rate modulation input and each operator is available for independent patching.
More info at Humble.audio