NodeDB is an online database which can help you find wi-fi access points , hot spots, fixed connections and even repeaters around you with results getting displayed in Google Maps. This service is free to everybody and you can even register your location with details of such as antenna description and pictures, SSID of the AP, even URLs to the usage policy of your location.
This way you can even start a group where you and your friends can join in and connect with each other. The best usage from my point of view is its really helpful when you are on the move and you are looking for a wi fi hot spot to connect to internet.
Ashish, thats a nice service, WeFi is another service which detects wifi within range, and also shows WiFi availability around the world.