Assume the following:
You are developing a server side application in GPL. Now this application serves HTML and not an executable which is directly executed on your machine. That means that another guy could take the GPL code, adapt it and does not necessarily need to publish it. Ie. he can create the identical service using your software without violating the GPL. (Although THEN he cannot publish the software itself i.e. selling)
Not so with the AGPL.
This hole in the GPL is often called "Application Service Provider" hole.
Search for "Why AGPL" or "AGPL vs. GPL" or just read this for some real projects who have problems with GPL. The MongoDB tries another interesing thing. They want that people do not fork the core DB (thatwhy AGPL) but the driver which has to be linked with the main programm is apache 2.0 licensed so that the mongoDB could be used within commercial application.
Public web application that uses the AGPL are listed at wikipedia.
