an aproach to http://www.ruby-forum.com/topic/3446541

enumerable_count_by.rb

$VERBOSE = true

module Enumerable
  def count_by
    return to_enum(__method__) unless block_given?
 
    Hash.new(0).tap{|h|
      each{|v|h[yield v] += 1}
    }
  end
end
 
list = [4, 5, 6, 4, 5, 6, 6, 7]
 
p list.group_by
p list.group_by{|v|v}
p list.group_by{|v|v * 2}
p list.count_by
p list.count_by{|v|v}
p list.count_by{|v|v * 2}

in ruby-1.9.3p0

Enumerator: [4, 5, 6, 4, 5, 6, 6, 7]:group_by
{4=>[4, 4], 5=>[5, 5], 6=>[6, 6, 6], 7=>[7]}
{8=>[4, 4], 10=>[5, 5], 12=>[6, 6, 6], 14=>[7]}

Enumerator: [4, 5, 6, 4, 5, 6, 6, 7]:count_by
{4=>2, 5=>2, 6=>3, 7=>1}
{8=>2, 10=>2, 12=>3, 14=>1}