The Light and Motion vis180 is a smooth pulse style of flasher which I think is what you are asking for. That said, I want a bright and aggressive flash - I think that is seriously beneficial in terms of visibility.

Dinotte (expensive) also has a mode that is a bright flash with some light in between (not full off). But it's flasher is good and bright and obnoxious - in other words, a perfect taillight that is highly visible.

J.