ruby怎么样能够探测到QQ是否在线？

不需要下载 mechanize-1.0.0.gem .
你只要执行 gem install mechanize 就OK.
如果你的 gem 从来没升级过, 需要先升级gem 很简单:

gem -v #看版本
gem update --system #升级, 升级完了就能 gem install mechanize

coding: utf-8

require 'rubygems'
require 'mechanize'

num = '51070540' #我的QQ

def online (num)
agent = Mechanize.new
agent.user_agent_alias = 'Linux Mozilla'
agent.max_history = 1
agent.open_timeout = 10
url = 'http://webpresence.qq.com/getonline?Type=1&' + num + ':'
agent.get_file(url)
end

puts online(num)

#你需要用 gem 安装 mechanize