For today only (at least for the next few hours) Payday: The Heist is free on Steam. You don’t need to download it. Simply clicking on “Install” will add it to your Steam library and you can then download at your leisure.
Steam is also holding a free weekend, with 10 games you can install and play for free over the weekend:
Payday 2
Company of Heroes 2
Awesomenauts
Don’t Starve
Trine 2
Grid 2
Killing Floor
Blade Symphony
XCOM: Enemy Unknown
Injustice: Gods Among Us
The games are also discounted over the weekend.