twat.rb

#!/usr/bin/env ruby
# Find common Twitter friends
# Usage: ./twat.rb user1 user2 ...

lists = ARGV.inject({}) do |hash, username|
  page = 0
  while !(output = `curl -s http://twitter.com/statuses/friends/#{username}.xml?page=#{page += 1} | grep "<screen_name>"`.gsub(%r{</?screen_name>}, '').split).empty?
    hash[username] = ((hash[username] || []) + output).sort
  end
  hash
end.values.inject { |a, b| a & b }.each { |user| puts user }

# Gems are nice too