Here is a quick/basic URI validation library for your Rails app:

require 'net/http'

# Original credits: http://blog.inquirylabs.com/2006/04/13/simple-uri-validation/
# HTTP Codes: http://www.ruby-doc.org/stdlib/libdoc/net/http/rdoc/classes/Net/HTTPResponse.html

class ActiveRecord::Base
  def self.validates_uri_existence_of(*attr_names)
    configuration = { :message => "is not valid or not responding", :on => :save, :with => nil }
    configuration.update(attr_names.pop) if attr_names.last.is_a?(Hash)
    
    raise(ArgumentError, "A regular expression must be supplied as the :with option of the configuration hash") unless configuration[:with].is_a?(Regexp)
    
    validates_each(attr_names, configuration) do |r, a, v|
        if v.to_s =~ configuration[:with] # check RegExp
              begin # check header response
                  case Net::HTTP.get_response(URI.parse(v))
                    when Net::HTTPSuccess then true
                    else r.errors.add(a, configuration[:message]) and false
                  end 
              rescue # Recover on DNS failures.. 
                  r.errors.add(a, configuration[:message]) and false
              end
        else
          r.errors.add(a, configuration[:message]) and false
        end 
    end
  end
end

Save the code above into a file in your 'lib' directory. ex: validates_uri_existence_of.rb. Include the file in your environment.rb (ex: require 'validates_uri_existence_of').

And now you can use the validator in any model by calling:

validates_uri_existence_of :url, :with =>
        /(^$)|(^(http|https)://[a-z0-9] ([-.]{1}[a-z0-9] )*.[a-z]{2,5}(([0-9]{1,5})?/.*)?$)/ix

Above code validates the URI/URL against a regular expression first, and then does a HEAD query to confirm that the page is alive (HTTPSuccess code is returned). You could easily modify the code to extend the functionality as you wish.

More info in this post.