Tropico 4 is free on the Humble Bundle Store as part of their summer sale celebrations. The game is a strategy simulation game, which lets you manage a banana republic as its El Presidente, a dictator. It's quite a fun game, and is widely regarded as the best in the Tropico series.
As a bonus Tropico 4 does have Steam Trading Cards, which you can sell on the Steam Community Market to get Steam Wallet Credits.
How to get a free Tropico 4 Steam Keyfrom Humble Bundle:
1. Visit the Tropico 4 store page on Humble Bundle's website. Login to your account or create a free one You will need to link your Steam account to your Humble Bundle account. If you have already done it, proceed to step 2.
2. Click on the "Get it free" button as shown in the screenshot above.
3. Proceed with the checkout process by clicking on the basket, and then confirm your order.
4. You will get a pop-up message which says " Click here for your steam key" , and you will also get an email saying the same.
5. You will get an option to redeem the key. But it actually just gives you the Steam Key. Copy the key and go to step 6.
6. To activate the game on Steam, open the Steam client and click on the "+ Add a Game" button, in the bottom left corner. Select "Activate a Product on Steam", a window will pop-up. Hit "Next" and "I agree to the terms".
7. Paste the Steam Key in the text box, and click on "Next" and "Finish" to add the game to your library.
This offer is only valid until Saturday, September 10th.
IMPORTANT:
You have to redeem the Steam key before September 17th, 2016 10:00 AM PST. Else, the key will be deleted from your Humble Bundle account.