Nokogiri (github): Co-author and maintainer of the premier Ruby XML/HTML parsing library. Fast, and with standards-compliant support for CSS and XPath.
Mechanize (github): Co-maintainer of the premier Ruby web scraping library.
Loofah (github): Author and maintainer of this HTML/XML manipulation and sanitization library, which is now the default sanitizer in Rails.
LicenseFinder (github): Maintainer of this tool to discover application dependencies and those dependencies' licenses.
ruby-ffi (github): Contributor to Ruby's Foreign Function Interface project, for JRuby and MRI.
chromedriver-helper (github): Author and maintainer of this tool to ease installation of chromedriver, the Chromium project's selenium webdriver adapter.
ActiveHash (github): Co-author of this Rails gem which builds ActiveRecord-like models from non-database, readonly data sources like a Hash, a YAML file, or any custom data file format you may have laying around.

McBean (github): Author of this nascent Ruby library whose goal is to transform documents seamlessly between markup formats including HTML, Markdown, and Textile.
Lorax (github): Author of this early-alpha (unstable) XML diff library for Ruby. "I am the Lorax, I speak for the trees."
flexible-js-formatting (github): Contributor to Baron Schwartz's super-fast javascript library for formatting and parsing datetimes and numbers. If you use Datejs, you will be blown away by how fast this library is, thanks to its badass use of badass javascript metaprogramming. Read Baron's original article for a taste. Badass!
Rinari: Ruby on Rails Minor Mode for Emacs: Contributor to Rinari, the most excellent rails mode for the most excellent editor.

Noteworthy Open-Source Projects