Sails: a dynamic, open-source web framework for Java designed in the spirit of Rails.
The top 5 reasons to use it?
1. Created using test-driven development (no, seriously, it was!)
2. Controllers are testable (oh yeah)
3. Leans heavily on convention over configuration (Rails anyone?)
4. Binds URLs to Actions on Controllers and renders their templates (sure wish Struts would do this sans XML)
5. Converts objects into Strings and Strings into objects (sure, Struts does this, but Sails does it better)
Born out of a software project some of my old co-workers from RoleModel Software are currently working on, this framework appears to be quite slick, but is still in its infancy. My experience with it so far has been limited, but I like what I see. I encourage you to at least poke around the Sails web site and read up on the framework.