Fancy a free copy of Portal? Well, Steam users on both PC and Mac can download and install the game for free up until September 20. The question that needs to be answered is: is it 100% free or simply a free trial? Well, you'll be happy to know that as long as you download and install it before September 20, it is free, indefinitely.
Valve is giving away Portal to help promote their "Learn with Portals" program, an education-based initiative to teach school-aged children about science. The program allows kids to design their own Portal levels, meaning they need to use physics, math, logic, probability and spatial reasoning to succeed.