Ruby on Rails is an amazing and elegant framework for developing web applications. I use it for many clients who are developing non-content-centric web applications and even some web apps that are content-centric but highly specialized.
As of Rails 3, it is now recommended that plugins are packaged as Ruby Gems (a structured, archival format similar to a zip file) and I've written a few of them:
Metafy – Metafy is a Gem for Rails 3 that makes it possible to easily add dynamic attributes to any ActiveRecord model.