Recently merb got some attention with her great features. It is available as a gem as well. Nice to hear from a great framework that is "ORM-agnostic, JavaScript library agnostic, and template language agnostic".
An other wanna-try is Camping, a super-lightweight framework from the creator of the amazing Hackety Hack toy bundle.
Obviously Nitro is a worth while framework. It supports the Ruby language community site and many others.