RailRoad

RailRoad is a Ruby application that generates model or controller diagrams for Ruby on Rails applications. It analyzes properties, inheritance, model associations, and so forth, and produces a graph description in the DOT language which can then be handled with third-party tools. The official site features several compelling examples, including model and controller maps of the Typo blogging system (as seen above).

Scruby

Scruby (not to be confused with the ScRUBYt! site scraping tool) is a Ruby application that works on UNIX-related operating systems and provides a shell where you can perform packet creation, sending and sniffing functions in a Ruby-esque fashion. It looks pretty powerful and the creator, Sylvain Sarmejeanne, has put together a very comprehensive page explaining how it all works.