Selenium implements the W3C WebDriver protocol to automate popular browsers. It aims to mimic the behaviour of a real user as it interacts with the application's HTML. It's primarily intended for web application testing, but any web-based task can automated.

Capybara is an integration testing tool for rack based web applications. It simulates how a user would interact with a website

Watir stands for Web Application Testing In Ruby It facilitates the writing of automated tests by mimicing the behavior of a user interacting with a website.

WebDriver-backed Watir

Headless Webkit driver for Capybara

SitePrism gives you a simple, clean and semantic DSL for describing your site. SitePrism implements the Page Object Model pattern on top of Capybara.

Poltergeist is a driver for Capybara that allows you to run your tests on a headless WebKit browser, provided by PhantomJS.

Celerity is a JRuby wrapper around HtmlUnit – a headless Java browser with JavaScript support. It provides a simple API for programmatic navigation through web applications. Celerity provides a superset of Watir's API.

Integrates cypress with rails or rack applications

Webrat lets you quickly write expressive and robust acceptance tests for a Ruby web application. It supports simulating a browser inside a Ruby process to avoid the performance hit and browser dependency of Selenium or Watir, but the same API can also be used to drive real Selenium tests when necessary (eg. for testing AJAX interactions). Most Ruby web frameworks and testing frameworks are supported.

Official Ruby Client for Selenium RC.

Headless Browser for Integration Testing with Capybara

Headless is a Ruby interface for Xvfb. It allows you to create a headless display straight from Ruby code, hiding some low-level action.

Cuprite is a driver for Capybara that allows you to run your tests on a headless Chrome browser

Culerity integrates Cucumber and Celerity in order to test your application's full stack.

Selenium RC with Rails integration and enhanced assertions.

Simple page objects for Capybara. All tooth-generated methods return Capybara Elements so that you can use these familiar objects for your needs.

Chrome driver for capybara using remote debugging protocol.

The "official" Selenium On Rails repository, synced with OpenQA.