This is my <100-line spec framework, and some accompanying demo code that showcases all the features.

spec_runner.rb

#!/usr/bin/env ruby

module Spec
	class AssertionError < Exception; end

	class Should
		def initialize(obj, invert=false)
			@invert = invert
			@self = obj
		end
		def ==(other)
			test = (@self == other)
			raise AssertionError unless (@invert ? !test : test)
		end
		def !=(other)
			@invert = !@invert; self == other; @invert = !@invert
		end
	end

	class MockObject
		def initialize(vars)
			@vars = vars
			@calls = Hash.new
			@accept_any_call = false
		end
		def method_missing(method, *a)
			@calls[method] = true if @calls.key? method
			return @vars[method] if @vars.key? method
			return if @calls.key?(method) or @accept_any_call
			raise "No method #{method}"
		end
		def should_receive(call)
			@calls[call] = false
		end
		def received?
			@calls.each_pair { |call,b| raise "Never got #{call}" if !b }
		end
		def as_null_object
			@accept_any_call = true
			self
		end
	end
	
	class TestCase
		attr_reader :subject

		def initialize(const,block)
			@const = const
			@tests = @passed_tests = 0
			@start_time = Time.now
			@exceptions = Array.new
			instance_eval &block
		end
		def it(desc)
			@subject = @const.new if @const.class == Class
			@mocks = Array.new
			@tests += 1
			begin
				yield
				@mocks.each { |m| m.received? }
				print '.'
				@passed_tests += 1 
			rescue Exception => e
				print 'F'
				@exceptions.push e
			end
		end
		def mock(properties)
			(@mocks.push Spec::MockObject.new(properties)).last
		end
		def test_info
			{ :tests => @tests, :passed => @passed_tests,
				:time => (Time.now - @start_time),
				:exceptions => @exceptions }
		end
	end
end

def describe(const, &block)
	info = Spec::TestCase.new(const, block).test_info
	puts "\n\n%d/%d tests passed in %0.5f seconds" % info.values[0..2]
	puts "Problems:\n" + info[:exceptions].map { |e|
		"- "+e.message+e.backtrace[0..6].select { |b|
			!(b =~ /spec/) }.map { |b| "\n"+(" "*4)+b }.join
	}.join("\n") unless info[:exceptions].empty?
end

class Object
	def should; Spec::Should.new(self); end
	def should_not; Spec::Should.new(self, true); end
end

ARGV.each { |file| require_relative file }

walrus_spec.rb

class Walrus
	def initialize
		@full = false
	end
	def give_gift(obj)
		if obj.edible then
			@full = true
			obj.digest!
		else
			obj.throw_away!
		end
	end
	def full?
		@full
	end
end

describe Walrus do
	it "starts its life hungry" do
		subject.full?.should != true
	end
	it "eats food" do
		milk = mock(:edible => true)
		milk.should_receive :digest!
		subject.give_gift(milk)
		subject.full?.should == true
	end
	it "doesn't eat non-food" do
		shoe = mock(:edible => false).as_null_object
		subject.give_gift(shoe)
		subject.full?.should_not == true
	end
end