Don Stephan
There has been a rich discussion of so many different possible causes, but I'm going to focus on the one that always causes ripples for me - too much pressure at the cutting edge onto the wood. I've made ripples with skew, spindle gouge, and bowl gouge, and I've always found too much pressure to be the cause. Stuart Batty has a nice demonstration of this on one of his videos on Vimeo.

The suggestion I've seen countless times is "ride the bevel" but a few videos I've seen have suggested instead "glide the bevel" or "float the bevel."

When I feel this vibration starting, either I stop the cut or I try to minimize the pressure of the cutting edge on the wood.