Game Maker 7

Here's the link to the YoYo Games web site where you can download the Game Maker program:
http://www.yoyogames.com/gamemaker/try

There are also some tutorials on the site which will help you to master programming your own games.

A great tutorial by an 11 year old boy on how to make platform games:
http://www.youtube.com/watch?v=SfdKS6NHjWI

You can find them here:
http://www.yoyogames.com/make/tutorials

Even better, here is a link to an electronic version of a brilliant book all about making games with Game Maker:
http://books.google.co.uk/books?id=4mXksWmedYYC&printsec=frontcover

The book normally costs about £30 in the shops! You would have to find some sprites to replace the ones used in the games in the book though :(

Have fun - be sure to bring in any games that you make at home.