InstantRails
Instant Rails is a one-stop Rails runtime solution containing Ruby, Rails, Apache, and MySQL, all preconfigured and ready to run. No installer, you simply drop it into the directory of your choice and run it. It does not modify your system environment.
Slightly unrelated but I have updated the PHP from 4 to 5.1.4 incase you might be running CakePHP or something else of the like in parallel with Rails.