A long time ago, this cool game called Portal came out.  It used the source engine and all that, and it’s premise was deceptively simple.  You can shoot this gun that will put a portal wherever you want it, then you put the other end of the portal wherever you want to go to.  Using this neat tool, you need to solve a series of room-base puzzles, in order to leave.
Now, you can play a great 2d version of it anywhere.  Well, anywhere you have a browser with Flash Player installed anyway.  Check it: http://portal.wecreatestuff.com/portal.php