Hobo: A Rapid Web App Builder for Rails
You may have thought Ruby on Rails was enough to get Web applications developed quickly, but enter Hobo. Hobo makes the process of creating Web applications and prototypes even quicker. For example, out of the box, with no lines of code written, you get a dummy app with a user signup, login, and authentication system.
Hobo can be installed as a gem so you can create new applications from scratch with the hobo command line tool, or as a plugin so you can add Hobo features to existing applications. Learn more in the installation guide. Hobo also includes a templating system and mark up language called DRYML (Don't Repeat Yourself Markup Language) that allows you to include things using custom defined HTML tags, rather than <%= %> blocks.
There's quite a lot to Hobo, so you'll want to go through its comprehensive official site and watch the Hobo screencast to get a real feel for it where a classified ads app is created within minutes.