brew/Library/Homebrew/cmd/leaves.rb

require 'formula'
require 'tab'
require 'set'

module Homebrew
  def leaves
    installed = Formula.installed
    deps_of_installed = Set.new

    installed.each do |f|
      deps = []
      tab = Tab.for_formula(f)

      f.deps.each do |dep|
        if dep.optional? || dep.recommended?
          deps << dep.to_formula.name if tab.with?(dep)
        else
          deps << dep.to_formula.name
        end
      end

      deps_of_installed.merge(deps)
    end

    installed.each do |f|
      puts f.name unless deps_of_installed.include? f.name
    end
  end
end