Very simple unit testing framework (in ~44 lines)

dsl.rb

class Dsl
  def initialize
    @tests = {}
  end
  def parse(description, block)
    self.instance_eval(&block)
    Executor.new(description, @tests)
  end
  def it(description, &block)
    @tests[description] = block
  end
end

executor.rb

class Executor
  def initialize(description, tests)
    @description = description
    @tests = tests
    @success_count = 0
    @failure_count = 0
  end
  def execute
    puts "#{@description}"
    @tests.each_pair do |name, block|
      print " - #{name}"
      result = self.instance_eval(&block)
      result ? @success_count += 1 : @failure_count += 1
      puts result ? " SUCCESS" : " FAILURE"
    end
    summary
  end
  def summary
    puts "\n#{@tests.keys.size} tests, #{@success_count} success, #{@failure_count} failure"
  end
end

ext.rb

module Kernel
  def describe(description, &block)
    tests = Dsl.new.parse(description, block)
    tests.execute
  end
end

class Object
  def should
    self
  end
end

sample.rb

require_relative "ext"
require_relative "dsl"
require_relative "executor"

describe "some test" do
  it "should be true" do
    true.should == true
  end
 
  it "should show that an expression can be true" do
    (5 == 5).should == true
  end
 
  it "should be failing deliberately" do
    5.should == 6
  end
end