Portal

Portal is a standalone game created from the world of Half-Life 2. You take role of a guy equipped with weapon similar to Gravity gun, which opens a portal when you shoot at a wall or floor. So it is a entertaining puzzle game. There are many levels, more and more difficult and many of them are fun for hours. New portals opening randomly, mini puzzles in many levels and more. Portal was first included with the release of Half-Life 2's Orange Box. Now, each game from that pack is available as its own boxed title for gamers to pick and choose their favorites.