Ruby - keep track of urls you've already visited

How do I keep track or urls that I've already visit in my ruby projects? Here's what I do:

def visited? url
  @visited ||= []
  return true if @visited.include? url
  @visited << url
  false
end

Now I've got a visited? method that tells me if I've called it already on an url

visited?('http://www.google.com')
#=> false
visited?('http://www.google.com')
#=> true

That makes it easy to just do something like:

scrape(url) unless visited?(url)

This was a short post but this is something I always put in my ruby code and I'll be referencing it later.