I just bought one of these puppies:
I would have preferred something cheaper, but I was at a bike store picking up some replacement brakes and my butt was still vibrating.
It cuts the vibrations a ton, when I'm actually sitting on the meaty part of the seat. I have a mountain bike, so the geometry keeps me towards the front of the seat. I need some handlebars that will reach closer to me.
... and it's got air-conditioning too.