Algorithm of Onslaught invented a new method of playing back sampled audio on the C64's SID chip. The strong points of this method are it's minimal rastertime usage and very good compression ratio.
FRODIGI (aka Free Running Oscillator DIGI) works on 8580 only. It is still work-in-progress, and does not yet perform well as a general purpose approach, as you can clearly hear in the demo ;) The results are impressive nontheless.
Anyway, check out the demo at csdb.