How many of these are widely used in production? Merb was close. But now its part of Rails. Leaving these 2 there aren't any in the Ruby world. I see Sinatra blinking somewhere. But its far far away.
The Merb/Rails merge reveals some reasons why there aren't a lot of widely used Ruby frameworks, and I'd say it's mostly a large scale manifestation of DRY, combined with the maturity and wealth of features in the Rails codebase and the fact that most of the changes devs would like to see in Rails are already going into it.