Hpricot is a very flexible HTML parser, based on Tanaka Akira's HTree and John Resig's JQuery, but with the scanner recoded in C (using Ragel for scanning.) I've borrowed what I believe to be the best ideas from these wares to make Hpricot heaps of fun to use....

Lightweight, fast and modular MVC Ruby web framework. Excellent for high performance dynamic pages and web services.

The OpenKomodo Open Source Development Environment for Dynamic Languages and Web Technologies

The Open Komodo project provides a code base upon which integrated development environment (IDE) software packages can be developed. ActiveState's Komodo Edit 4.3 and later (an open source, multi-platform, multi-language editor) is an established, mature product that uses the Open Komodo platform....

RSpec is a BDD tool and a framework which provides programmers with a Domain Specific Language to describe the behaviour of Ruby code with readable, executable examples that guide you in the design process and serve well as both documentation and tests.

Sinatra a DSL for quickly creating web-applications in Ruby with minimal effort.,

Phusion Passenger — a.k.a. mod_rails — makes deployment of applications built on the revolutionary Ruby on Rails web framework a breeze. It follows the usual Ruby on Rails conventions, such as “Don’t-Repeat-Yourself”.

Note that the Ohloh source analysis is incorrect. The statistics for C++ and comment count are wrong because the Boost library is included. It is also not true that there's a single developer, because Passenger is commercially supported by Phusion....

RubyGems is a package manager for the Ruby. It provides a standard format for distributing Ruby programs and libraries (in a self-contained format called "gems"), a tool designed to easily manage the installation of gems, and a server for distributing them....

Puppet lets you centrally manage every important aspect of your system using a cross-platform specification language that manages all the separate elements normally aggregated in different files, like users, cron jobs, and hosts, along with obviously discrete elements like packages, services, and files....

Rake is a Ruby make-like utility using standard Ruby syntax for the build specification file.

Cross platform BDD tool that lets domain experts define software behaviour in plain text. The text can be executed as automated tests.