Know Everything about your network and computer network configuration -- Cool tool, since long I am watching the application which tell the domain to IP mapping, IP to area mapping and then "whois" tells the complete info about the administrator and manager of one's network.
Then this tool, named BugMeNot.com was created as a mechanism to quickly bypass the login of web sites that require compulsory registration and/or the collection of personal/demographic information (such as the New York Times).
In the FAQ of the same site I found the answer to question which most of us want "I want to create some fake accounts but I need a valid email account." :D :D
checkout => http://bugmenot.com/faq.php#12
Really cool "secure" stuff to peep into others not allowed knowledge base and information :)
EDITED: 22 August, 2005
Here is one more namely, GVisit - a superb tool which tells the location of last "20" (or more in case you are ready to pay) visitors to your webpage with time using GoogleMaps.
A good GUI is there to make it better. Technology doesnot seems much hard/tough. Just needs the IP of the person accessing ur SITE which can be known peacefully, the map it using existing Databases to the location. and using Google Maps to present it.
But getting the idea first time and building a good worth using tool is an art.