So please, next time someone googles selenium rails, let them know that Capybara is there to help, willya?
P.S. If you want to run against any web server on the internet, without any special support for a local Rack/Rails stack, you could also use the official selenium-webdriver gem directly. (Though to be fair, Capybara supports running against remote servers too, if you need it.)
P.P.S. John Firebaugh provides some more detail on the confusing state of the Selenium Ruby libraries.