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In additional to Skyrim (already posted) and AOE (expired), these are some of the best Amazon deals. I believe they are all Steam codes.
Sword of the Stars The Pit GOLD (this includes all the DLCs) is $1.75 http://www.amazon.com/Sword-Stars-Gold-Online-Game/dp/B00H7M…
The base game is $1.75 on Steam and almost $6 with the DLCs. Another similar type of game is Dungeons of Dredmore, which is cheap but only 50% off on Steam.
Payday 1 The Heist $1.49 (93% off) http://www.amazon.com/Sony-40814Y-The-Heist1-Download/dp/B00…
Takedown Red Sabre $1 (generally bad reviews) http://www.amazon.com/TAKEDOWN-Sabre-Online-Game-Code/dp/B00…
How to Survive $3.75 (is regularly on sale) http://www.amazon.com/How-Survive-Online-Game-Code/dp/B00FY3…
I am half way through How to Survive, and rather enjoy it. Crafting and Undead are both trendy, so why not combine the 2.
State of Decay $9.99 (50% off) http://www.amazon.com/State-Decay-Online-Game-Code/dp/B00GU0…
Your link for State Of Decay leads me to $9.99 (50% off)