Wikipedia
WEBrick
WEBrick is a Ruby library providing simple HTTP web server services. WEBrick was primarily written by Masayoshi Takahashi and Yuuzou Gotou, with contributions from other developers via the open source model of software development. The server provides HTTP, HTTPS with authentication, setup of Servlet with Ruby code, and other features.
It is used by the Ruby on Rails and Padrino frameworks to test applications in a development environment as well as production mode. It is now part of Ruby standard library for Ruby 1.9.3, and updated with Ruby trunk. The documentation is incomplete for some modules and methods.