Overdrive, with 4x oversampling, in core.
Toob-ular overdrive effect unit, with 4x oversampling to reduce aliasing effects. Nice warmth at moderate drive settings; heavy distortion, at high drive settings, but with much less of that harsh digital nastiness.
I didn't think oversampling would make that much difference; but it did.
If you're looking for core macros to use for a distortion/overdrive unit in a synth, the core macro around which this effect is built may be a good choice.
The basic premise: convert to 4x oversampling (via lagrange interpolation), waveshape, low-pass filter at about 20khz, and downsample via decimation. This reduces SOME of the aliasing introduced by wave-shaping. Just enough to make it sound a little sweeter than it would with straight waveshaping. Might be a useful approach to explore with more interesing non-linear manipulation.
v1.2: Waveshape graph, better range on the drive control, and gratuitous de-beautification.