extremetracker or use this <p>There are currently <script type="text/javascript" src="http://fastonlineusers.com/on4.php?d=www.yoursiteaddress"></script> users online</p>
Hmm i think the op ascks of a service where you enter a url (competition probably) and it tells you how much visitators it has Hmmm such url dioesn't exist and i don't see how somebody to make one it's impossible ...
You should check out Compete.com It allows you to see estimated traffic to any URLs that you enter and compare up to 3 results at once. ie. your site vs. your competitors. You can also find out things like what keywords were used to get to your competitor's site as well as other cool things.