Merb (Mongrel plus EeRuby) is an MVC (ModelViewController) framework.
For more information see http://merbivore.com/index.html
Formerly "Recommended as an alternative to RubyOnRails on RubyOnRailsRulesTheUniverse."
Now it is mandated as the alternative - Rails3 will use a Merb core: MerbAndRailsMerge. The dark secret here was Ruby and TDD were so fine that Rails could become Ruby's KillerApp despite having a weak core. Merb is all about core, pluggability, and SoftwareProductLines, so RailsThree? is going to be just another of its products.
Here's some Merb, and GraphViz, showing a mind-map of a blog:
CategoryRuby | CategoryFramework