Ruby Mechanize: пользовательские агенты?

да. Посмотрите на https://github.com/sparklemotion/mechanize/blob/master/lib/mechanize.rb#L115:

AGENT_ALIASES = {
  'Windows IE 6' => 'Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1)',
  'Windows IE 7' => 'Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; .NET CLR 1.1.4322; .NET CLR 2.0.50727)',
  'Windows Mozilla' => 'Mozilla/5.0 (Windows; U; Windows NT 5.0; en-US; rv:1.4b) Gecko/20030516 Mozilla Firebird/0.6',
  'Mac Safari' => 'Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10_6_2; de-at) AppleWebKit/531.21.8 (KHTML, like Gecko) Version/4.0.4 Safari/531.21.10',
  'Mac FireFox' => 'Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.6; en-US; rv:1.9.2) Gecko/20100115 Firefox/3.6',
  'Mac Mozilla' => 'Mozilla/5.0 (Macintosh; U; PPC Mac OS X Mach-O; en-US; rv:1.4a) Gecko/20030401',
  'Linux Mozilla' => 'Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.4) Gecko/20030624',
  'Linux Firefox' => 'Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.2.1) Gecko/20100122 firefox/3.6.1',
  'Linux Konqueror' => 'Mozilla/5.0 (compatible; Konqueror/3; Linux)',
  'iPhone' => 'Mozilla/5.0 (iPhone; U; CPU like Mac OS X; en) AppleWebKit/420+ (KHTML, like Gecko) Version/3.0 Mobile/1C28 Safari/419.3',
  'Mechanize' => "WWW-Mechanize/#{VERSION} (http://rubyforge.org/projects/mechanize/)"
}

Вы можете проверить пользовательские агенты Mechanize с помощью этого кода:

require 'rubygems'
require 'mechanize'
pp Mechanize::AGENT_ALIASES

и с текущей версией он выведет:

{"Mechanize"=>
  "Mechanize/2.7.0 Ruby/1.9.3p429 (http://github.com/sparklemotion/mechanize/)",
 "Linux Firefox"=>
  "Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.2.1) Gecko/20100122 firefox/3.6.1",
 "Linux Konqueror"=>"Mozilla/5.0 (compatible; Konqueror/3; Linux)",
 "Linux Mozilla"=>
  "Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.4) Gecko/20030624",
 "Mac Firefox"=>
  "Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.6; en-US; rv:1.9.2) Gecko/20100115 Firefox/3.6",
 "Mac Mozilla"=>
  "Mozilla/5.0 (Macintosh; U; PPC Mac OS X Mach-O; en-US; rv:1.4a) Gecko/20030401",
 "Mac Safari 4"=>
  "Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10_6_2; de-at) AppleWebKit/531.21.8 (KHTML, like Gecko) Version/4.0.4 Safari/531.21.10",
 "Mac Safari"=>
  "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_7_2) AppleWebKit/534.51.22 (KHTML, like Gecko) Version/5.1.1 Safari/534.51.22",
 "Windows IE 6"=>"Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1)",
 "Windows IE 7"=>
  "Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; .NET CLR 1.1.4322; .NET CLR 2.0.50727)",
 "Windows IE 8"=>
  "Mozilla/5.0 (compatible; MSIE 8.0; Windows NT 5.1; Trident/4.0; .NET CLR 1.1.4322; .NET CLR 2.0.50727)",
 "Windows IE 9"=>
  "Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; Trident/5.0)",
 "Windows Mozilla"=>
  "Mozilla/5.0 (Windows; U; Windows NT 5.0; en-US; rv:1.4b) Gecko/20030516 Mozilla Firebird/0.6",
 "iPhone"=>
  "Mozilla/5.0 (iPhone; U; CPU like Mac OS X; en) AppleWebKit/420+ (KHTML, like Gecko) Version/3.0 Mobile/1C28 Safari/419.3",
 "iPad"=>
  "Mozilla/5.0 (iPad; U; CPU OS 3_2 like Mac OS X; en-us) AppleWebKit/531.21.10 (KHTML, like Gecko) Version/4.0.4 Mobile/7B334b Safari/531.21.10",
 "Android"=>
  "Mozilla/5.0 (Linux; U; Android 3.0; en-us; Xoom Build/HRI39) AppleWebKit/534.13 (KHTML, like Gecko) Version/4.0 Safari/534.13",
 "Mac FireFox"=>
  "Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.6; en-US; rv:1.9.2) Gecko/20100115 Firefox/3.6",
 "Linux FireFox"=>
  "Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.2.1) Gecko/20100122 firefox/3.6.1"}