It has a powerful building engine that lets you touch every conceivable part of the game and customize it down to the smallest detail. At first, the interface is a little overwhelming but the team has a tutorial feature that will help you a lot. Features are put up every other day, so you will have plenty of stuff to do. The site also has a forum, that is full of helpful people. They host competitions for all kinds of things as well. If you are not feeling like making a game, you can just browse through games made by others!

The main site- http://mygamebuilder.com/
The game making site- http://s3.amazonaws.com/apphost/MGB.html
The forum- http://mygamebuilder.com/forum/