Overkill Software has made Payday 2 temporarily free a few times in the past—trial weekends, a free week, that sort of thing. But now, while supplies last (only five million copies available!), it and Payday 2 VR are straight-up, keep-them-forever free on Steam.
The studio announced last month that Payday 2 and its great abundance of DLC would be consolidated into a single "Ultimate Edition" package. The all-in version of the game will go $45, while existing owners will have the option to upgrade, with pricing based on how much DLC they already have.
The changeover process is still underway, however, so while the existing DLC has all been removed, the Ultimate Edition is not yet available for purchase. So while that all grinds through the system, Overkill has made the base game temporarily free for the taking.
A freebie Q&A thread is available here if you have questions, but it's something of a goat rodeo, as you might expect—it seems that some people aren't happy that a game they paid for in 2014 is now free. An easier source of answers can be found at the Payday 2 Ultimate Edition FAQ.