More and more educators are embracing the math and science in modern games to help with learning in the class room and get kids engaged. Even if your teachers don't have an after school Minecraft class (more on that later), or you are a bit beyond the age for public school, doesn't mean you can't enjoy this fun learning environment. This weekend, until the 20th, Valve is offering their award winning game, Portal, for free. If you love it I recommend buying the sequel! Check out Learning with Portals, for more information. I personally have a Steam account and it's not intrusive at all.
This is a first person computer game that changes the way you think about space. By using portals you can move through space in a unique manner. Walls don't simply become doors to a new room, but can change the way you access the room you are already in. With all my projects and new games from the Humble Bundles I haven't gotten very far yet, but not because it's not an intriguing game, it is!
EDIT: You do not need to be an educator to reap this deal. It's free for anyone!
Also links added so Orcrist can stop whining about not enough links :)