domains.rb

#!/usr/bin/env ruby

domains = {}

ARGF.each do |email|
  domain = email.split("@").last.strip
  if domains[domain]
    domains[domain] += 1
  else
    domains[domain] = 1
  end
end

sorted = Hash[domains.sort_by{|k, v| v}.reverse]

sorted.each do |domain, count|
  puts "#{domain}, #{count}"
end