assert_select is a plugin by Assaf Arkin that allows you to use CSS selectors in your functional tests to check that certain HTML elements match your assertions. On the surface, this isn't too far removed from using why's Hpricot to do assertions on HTML, but in reality having the full power of CSS selectors available changes everything (Update! Hpricot has XPath and CSS addressing too!). Some examples:

# Form includes four input fields
assert_select "form input", 4

# Page does not have any forms in it.
assert_select "form", false, "Page must contain no forms"

def test_login_form_has_all_fields
    get :login
    assert_select "form[action=http://myapp/login] input" do |inputs|
        assert_equal 3, inputs.size
        assert_select inputs[0], "input[type=name][name=username]"
        assert_select inputs[1], "input[type=password][name=password]"
        assert_select inputs[2], "input[type=submit][value=Login]"
    end
end

More examples here.