Rack provides a minimal, modular and adaptable interface for developing web applications in Ruby. By wrapping HTTP requests and responses, it unifies the API for web servers, web frameworks, and software in between into a single method call.

Rack::Mount supports Rack's X-Cascade convention to continue trying routes if the response returns pass. This allows multiple routes to be nested or stacked on top of each other.

Oj is a JSON parser and generator for Ruby, where the encoding and decoding of JSON is implemented as a C extension to Ruby.

This module adds support for the TomDoc documentation format to YARD, a documentation generation tool for Ruby.

Prawn-dev contains tools to aid the development of the various Prawn projects.

Sentry provides a gem that provides a client interface for the Sentry error logger.

This library provides a way to run code around tests in Minitest, written using either the unit test or spec style.

This package can speed up Test::Unit, RSpec, Cucumber, and Spinach tests by running them concurrently across multiple CPU cores.

Minima is a one-size-fits-all Jekyll theme for writers. It's Jekyll's default (and first) theme. It's what you get when you run jekyll new.

Knows about MRI, Rubinius, JRuby, MagLev and MacRuby.

This package allows loading, parsing and cascading Cascading Style Sheets (CSS) rule sets in Ruby.

Clamp provides an object-model for command-line utilities. It handles parsing of command-line options, and generation of usage help.

Psych is a YAML parser and emitter. Psych leverages libyaml for its YAML parsing and emitting capabilities. In addition to wrapping libyaml, Psych also knows how to serialize and de-serialize most Ruby objects to and from the YAML format.

websocket-extensions provides a container for registering extension plugins.

This package provides a performant priority queue in pure ruby with support for changing priority using pairing heap data structure

ruby-unf is a wrapper library to bring Unicode Normalization Form support to both Ruby and JRuby. It uses unf_ext on CRuby and java.text.Normalizer on JRuby.

minitest/reporters provides a custom Minitest runner to improve how the test state is reported. A number of different reporters are available, including a spec reporter, progress bar reporter, a HTML reporter.

pkg-config can be used in your extconf.rb to properly detect need libraries for compiling Ruby native extensions.

Concurrent Ruby includes concurrency tools such as agents, futures, promises, thread pools, actors, supervisors, and more. It is inspired by Erlang, Clojure, Go, JavaScript, actors, and classic concurrency patterns.

EmailReplyTrimmer is a Ruby small library to trim replies from plain text email.

Skinny is a simple, upgradable WebSockets for Ruby, using the Thin library.

This package provides a Ruby wrapper around the Terraform command line interface so that Terraform can be more easily invoked from Ruby code.

Patron is a Ruby HTTP client library based on libcurl. It does not try to expose the full power (read complexity) of libcurl but instead tries to provide a sane API while taking advantage of libcurl under the hood.

m stands for metal, a better test/unit and minitest test runner that can run tests by line number.