GameMaker is a wonderful drag and drop gamemaking tool. It is easy enough for elementary students to use to create games and robust enough for the hard core programmer. I have used this to teach programming concepts and constructs with students from grade 2 through 10. I am continually amazed by the creations of my students and the countless hours they put into creating their games. The toolset is openended and provides the game designer with the opportunity to create any type of game they can think of without limitation.
GameMaker is supported by an online community called yoyogames (http://www.yoyogames.com/). The site allows you to download a plethora of resources. The program is available for free and an additional set of tools can be unlocked by purchasing the Pro version at a nominal fee. The pro version is not necessary, but for the small donation it is certainly worth supporting the development team. You can also download resources including tutorials, graphics, sounds, etc. Finally, you can post your games to the yoyogames site for others to enjoy or download games created by others.