If you own a PC or a Mac and enjoy bending your brain, then chances are that you have played Valve's groundbreaking Portal.  If you haven't, then now is the time to rectify that situation because for the next few days, Portal is free.  Just head on over to Steam and snag it by September 20 and it'll be linked to your account forever.  Pretty good deal, right?  PC Gamer explains why this is happening now:

It’s all part of Valve’s Learn With Portals initiative, which aims to promote Portal’s reality bending puzzles as an educational tool, and hopes to encourage the next generation to start building a new wave of even more dastardly test chambers. You can see our future tormentors learning the basics at Valve HQ in the video above. You can build your own levels with the free Portal authoring tools, which you’ll find the “tools” section of your Steam library.

There you have it.  No more excuses.  Just give it a try.  Trust me; you'll love it.  Meanwhile, the free downloadable expansion for the sequel, Portal 2, is still supposedly due out this month.