http://spyspace.standoffish.com
They are closed to new registrations now due to the overwhelming popularity but...basically you put a URL in your profile that loads a .SWF Flash ActionScript file everytime someone visits your site. That ActionScript file reports back to the page The Name/ID of the MySpace visitor...the referring URL (how they found your site), and the IP of the visitor.
I had fun with this for awhile but it actually made me delete my myspace account...i had girls that i know's boyfriends visiting my page on a daily basis.
Any clue on how this ActionScript might work? Decompiling it is useless...people have tried and it is protected.