Now, if you're one of those people who won't touch a game because you can't play it for 6 months straight then you might want to leave right now. Just letting you know. :)
FTL (Faster Then Light)
My brother has been addicted to this game and bought it for me. I refused to play it as it looked stupid. I have to say, I was oh so wrong. This game is so fun.
You chose your space ship and head through a number of galaxy's. At the end you face a big mother ship. Along the way you come across many obstacles, decisions and crazy battles. Seriously buy and give this a go. Its very cool and addictive.
Mark of the Ninja
I installed and played this because it was on sale. Holy cow! I'm a ninja! No, really, you actually feel like a ninja. Stealth, assassinations, the way you move, its very cool. This game has very high ratings. The cartoons between acts are well done too.
If you don't mind side scroller games like Trine (another awesome game) and like violence and cartoons, give this a look. I made level 3 and I can't wait to play it again.
Now you prob won't play these games for months but frankly, I find the longer the games life, the more drawn out and repetitive it is anyway. Sometimes its good to get away from the grind and just do something different.
Sigh, now I gotta go to work.