サンプルコード

require 'open-uri'
open('http://www.example.com/', {
  :method           => :post,
  :body             => 'a=123&b=456',
  'User-Agent'      => 'Mozilla/5.0 (Windows; U; Windows NT 5.1; ja; rv:1.8.1.12) Gecko/20080201 Firefox/2.0.0.12',
  'Accept'          => 'text/xml,application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5',
  'Accept-Language' => 'ja,en-us;q=0.7,en;q=0.3',
  'Accept-Charset'  => 'Shift_JIS,utf-8;q=0.7,*;q=0.7',
  'Referer'         => http://www.example.com/}) do |line|
    puts line.read
  end