Chip synth with 2 oscillators
SE64 by Oli Larkin is a free monosynth with 2 oscilators - one can be set to saw, triangle, sine or a variety of PWM waveforms.
The other oscillator is a triangle wave which is ring modulated with oscillator one.
The SE64 chip synth has a fast arpeggiator to recreate the SID chip sound of the Commodore 64, 4 envelopes:
pitch, pulse width, amplitude, and filter cutoff, and two bit-crusher effects.