Most noticeable Free Games giver is, probably, a Humble Bunde. They occasionally (at least once a month) gives out newer or older Games for everyone, who is registered at their platform. After the CD-key is received, it can be activated on your personal Steam account.
Ubisoft also has secret website, for Free offers they are having, at the moment. It might not be the Games that can be kept forever, but most likely a trial versions. Which lets to try the Game for Free and then buy it with a Discount. If player want to keep it. This requires to have Uplay client downloaded and installed.
Sometimes it is worth to look around in specially created Free Steam Games sites or Reddit thread, there users always share their findings, across the internet. But there are many different Games, not only PC, also: Mobile, indie and so on.