This is how I got my first set of fast tests running. There are probably a dozen ways to do it better, but this worked for me. I wanted to test some rails helper methods that had simple interfaces and no dependency on rails. Here's how I finally got it working.

gistfile1.rb

# spec/helpers/my_helper_spec.rb
# note: must replace references to my_helper/MyHelper to appropriate name

# require any dependencies - I needed date/time so just included all of active support
require 'active_support/all' 

# require the specific helper module
require_relative "../../app/helpers/my_helper.rb"

class DummyClass
end

describe MyHelper do

  before(:each) do
    @helper = DummyClass.new
    @helper.extend(MyHelper)
  end

  it "should return something" do
    @helper.some_method.should eq "something"
  end
    
  # ... more tests

end