Portal 2
Since Portal comes from Valve it is not that rare to find the game on sale on Steam. Yet with the Steam Summer Sale Portal is almost being offered for free. The tremendous puzzle platformer is discounted at 90% off so Portal 2 can be picked for the exact same number of dollars that are in the title. Portal 2 is worth any price but at 1.99 it is an absolute steal.
It is not necessary to play the first Portal to jump into Portal 2 (though the original game is also discounted for just 99 cents). The only thing that is truly required for Portal 2 is to put your gamer thinking cap on and immerse yourself in a strange, but hilarious, world of physics-based puzzles. Each challenge in Portal 2 builds on the previous obstacle allowing the player to feel like an absolute super genius by the end, even if they are just sitting in front a computer screen with a mouse.
Portal 2 manages to take all the charm that is found in the original game and up the ante. GLaDOS is still involved but she is joined by the somehow equally fascinating robot of Wheatley as they both take silent protagonist Chell on one very memorable adventure. Portal 2 is everything a sequel should be and an almost perfect video game experience blending story and gameplay seamlessly.