Making good use of polymorphism I had this idea for assertion methods:

  class TrueClass
    # Assert true.
    #
    #   (x == y).true!
    #
    def true!
      true
    end
    # Assert false.
    #
    #   (x == y).false!
    #
    def false!
      raise "not false!"
    end
  end

  class FalseClass
    # Assert true.
    #
    #   (x == y).true!
    #
    def true!
      raise "not true!"
    end
    # Assert false.
    #
    #   (x == y).false!
    #
    def false!
      true
    end
  end

If only the error message could say something about WHAT was not true
or false, this could be pretty nifty.