The incomparable "Butt Buddy". Lame name, but it pretty much does what it says on the tin. The elastomers give almost 1 cm of damping - this won't be enough for offroad riding, but it does noticably reduce road vibration and smaller bumps. I can't notice it flexing much when I pedal at high cadence either.

If I were starting from scratch, I might have prefered a sprung Brooks saddle. But that would have been fairly expensive. I got my titanium rail saddle for $20 secondhand, and the Butt Buddy was on sale for $25. If that isn't a bargain, I don't know!