The evercookie

I just learned about the evercookie – a test cookie designed to demonstrate how persistent cookie-type data can be in a browser, especially as HTML5 comes of age.

evercookie test (requires javascript)
NY Times Article

I recommend visiting the evercookie site with javascript enabled, click the Create button, then delete your browsers cache and cookies, close it, and reload it. Visit the evercookie site again and ‘Click to rediscover’. If it comes up with ‘undefined’ then your cleaning worked. If it returns a uid number then the cookie data is still present.