Grabs a proxy list from spys.ru and programmatically checks for the first one that responds fast enough for your liking.

get_proxy.rb

require 'net/http'
require 'timeout'

$proxies = nil
Net::HTTP.start('spys.ru') do |http|
  response, body = http.get('/en/http-proxy-list/')
  secret = body.match(/">(.+?);<\/script>/)[1].split(';')
  keys, holder = {}, binding
  secret.each { |s| keys[s.split('=')[0]] = eval(s, holder) }
  finder = %r{4>([0-9.]+)<.+?>"\+(.+?)\)<.+?/anon.+?[12]>(.+?)</f}
  $proxies = body.scan(finder)
  $proxies.each_with_index do |p, i|
    port = '', holder
    p[1].split('+').each { |k| port << eval(k, holder) }
    $proxies[i] = "#{p[0]}:#{port[2..-1].join}"
  end
end

def get_proxy
  return nil if $proxies.empty?
  Timeout::timeout(1) do
    test = $proxies.shift
    ip, port = test.split(':')
    Net::HTTP::Proxy(ip, port).start('www.google.com') do |http|
      response, body = http.get('/')
      return get_proxy unless ['200', '302'].include?(response.code)
    end
    return test
  end rescue get_proxy
end

get_proxy_wc.rb

require 'net/http'
require 'timeout'

$proxies = nil
# bad global, but it prevents doing the search again for each new test

Net::HTTP.start('spys.ru') do |http|
  response, body = http.get('/en/http-proxy-list/')
  secret = body.match(/">(.+?);<\/script>/)[1].split(';')
  # ex: g7v2=2677;t0z6=2189;f6e5=1478;q7z6j0=0^g7v2;l2o5e5=1^t0z6;h8n4p6=2^f6e5;
  # spys.ru seem like cool people; they offer their service entirely free,
  # and most of the proxies are pretty decent, but they do make it slightly
  # tougher to programmatically get the list
  # rather than just showing the ip:port combination in plain text, they're
  # using javascript, secret variables, and a bunch of xor all over the place
  # to eventually display the port
  # I'm sure they did this for a bit of "security", of course, but I also feel
  # like they were issuing it as a playful challenge to fellow programmers
  # maybe not, but I enjoyed decrypting them nonetheless

  keys, holder = {}, binding
  # keys holds the aforementioned variable names, holder is a binding for eval
  secret.each { |s| keys[s.split('=')[0]] = eval(s, holder) }
  finder = %r{4>([0-9.]+)<.+?>"\+(.+?)\)<.+?/anon.+?[12]>(.+?)</f}
  # pretty tame as far as regexes go; pulls out the ip, the javascript for
  # writing the xored port and the anonymity of the proxy; non-anonymous proxies
  # are silly, so filter them
  # ex: ['85.114.132.49', '(y5t0y5^w3t0)+(j0p6j0^g7i9)+(d4c3f6^v2c3)+(b2y5o5^l2d4)', 'ANM']

  $proxies = body.scan(finder)
  $proxies.each_with_index do |p, i|
    port = '', holder # same eval binding, of course
    p[1].split('+').each { |k| port << eval(k, holder) }
    # the addition in the javascript isn't numeric, so evaling the whole thing
    # won't do; that's why port is initialized as an empty string
    $proxies[i] = "#{p[0]}:#{port[2..-1].join}"
    # mush it all back together, ex: 85.114.132.49:3128
    # start from index 2 because 0 is '' and 1 is holder.class (Binding)
  end
end

def get_proxy
  return nil if $proxies.empty? # bad luck, not a single proxy qualified
  Timeout::timeout(3) do
    test = $proxies.shift
    ip, port = test.split(':')
    Net::HTTP::Proxy(ip, port).start('www.google.com') do |http|
      response, body = http.get('/')
      # the proxy server might have responded quickly, but can you use it?
      return get_proxy unless ['200', '302'].include?(response.code)
      # 200 for OK, 302 for the redirect Google sends for other countries
    end
    return test
  end rescue get_proxy # timeout expired, try again
end