Option #1 Selenium-WebDriver gem

First install the selenium-webdriver gem:

gem install selenium-webdriver

Then start using it:

require 'selenium-webdriver'
width = 1024
height = 728
driver = Selenium::WebDriver.for :firefox
driver.navigate.to 'http://google.com'
driver.execute_script %Q{
  window.resizeTo(#{width}, #{height});
}
driver.save_screenshot('/tmp/screenshot.png')
driver.quit

Reference.

Option #2 Selenium Server and the Selenium Gem

I was using this option for quite some time, when it stopped working I started using the selenium-webdriver gem instead.

First create a separate Firefox profile:

firefox -ProfileManager -no-remote

Find the path to the profile by executing this code in the JavaScript error console in Firefox:

Components.classes["@mozilla.org/file/directory_service;1"].getService( Components.interfaces.nsIProperties).get("ProfD", Components.interfaces.nsIFile).path

The path looks somewhat like this on OSX:

/Users/christian/Library/Application Support/Firefox/Profiles/xxx

Now download the Selenium Server. Then start a Selenium server:

java -jar ~/Downloads/selenium-server-standalone-2.0b1.jar -firefoxProfileTemplate "/Users/christian/Library/Application Support/Firefox/Profiles/9eb0mn2d.screenshots/" -singleWindow

Next install the Selenium Ruby client:

gem install selenium-client

Take a screenshot with Ruby:

require 'rubygems'
require 'selenium'

@selenium = Selenium::SeleniumDriver.new("localhost", 4444, "*firefox", "http://www.cnn.com", 10000);
@selenium.start
@selenium.open "/US/"
@selenium.wait_for_page_to_load(60)
@selenium.send(:sleep, 7) # Wait for Flash and other crap to load
@selenium.capture_entire_page_screenshot('/tmp/screenshot.png', '');
@selenium.stop

Want more? Read the Selenium RC and Selenium client documentation