Haml-rails provides Haml generators for Rails 3. It also enables Haml as the templating engine for you, so you don't have to screw around in your own application.rb when your Gemfile already clearly indicated what templating engine you have installed. Hurrah.

Git history statistics generator

A gem for generating embeddable HTML previews from URLs.

Provides a dev bar and an overlay in-browser to visualize your UI's rendered partials

Deface is a library that allows you to customize ERB, Haml and Slim views in a Rails application without editing the underlying view.

A server that stores and returns pact files generated by the pact gem. It enables head/prod cross testing of the consumer and provider projects.

Nesta is a lightweight Content Management System, written in Ruby using the Sinatra web framework. Nesta has the simplicity of a static site generator, but (being a fully fledged Rack application) allows you to serve dynamic content on demand. Content is stored on disk in plain text files (there is no database). Edit your content in a text editor and keep it under version control (most people use git, but any version control system will do fine). Implementing your site's design is easy, but Nesta also has a small selection of themes to choose from.

Open-source Dropbox using Ruby and Git.

Automatically generate beautiful front-end style guides with Sass and Markdown. See https://livingstyleguide.org for details.

Serve is a small Rack-based web server that makes it easy to serve ERB or HAML from any directory. Serve is an ideal tool for building HTML prototypes of Rails applications. Serve can also handle SASS, Textile, and Markdown if the appropriate gems are installed.

dawn is a security source code scanner for ruby powered code. It is especially designed for web applications, but it works also with general purpose ruby scripts. Dawn supports all major MVC frameworks like ruby on rails, padrino and sinatra; it provides more than 680 security checks with their own mitigation suggestion.

Codesake::Dawn is a security source code scanner for ruby powered code. Starting from January 07, 2015 this gem is renamed to dawnscanner and this version is no longer supported. Please, upgrade your Gemfile.

Application registry inspired by Apache Zookeeper

Easily style buttons with beautiful CSS3 features like gradients, rounded corners, etc. Don't worry the buttons also degrade nicely for browsers with no support. This requires the Compass stylesheet authoring framework. Forked, to handle text colors better.

A time-based background job scheduler daemon (just like Cron) for Rails

Package your assets for Sinatra.

Web component framework for Rails providing widgets that trigger events and know when and how to update themselves with AJAX.

Phrasing!

Generate cheat sheets for Dash

This gem helps create large (>50) gmail filter chains by writing xml compatible with gmail's "import/export filters" feature.