I have a few games I'd like to mention here:
Team Fortress 2. I played TF2 for a few years on and off and I cherish the memories I have with it, even including fairly recent ones like when I played Pyro Shark for 3 hours on a 2Fort server The game lacks in no place aside from actual attention from Valve (and the balancing isn't really perfect) and it's really showing that even with now 4 years without a major update that people still flock to this game even now.
Paladins: Champions of the Realm. I'd say this game is a decent ways away from being as good as TF2, but I keep being drawn to the (unfortunately less than before) amount of depth this game offers. Even if I don't necessarily like certain aspects of the game (like its meta right now and its community of uhhh not entirely great people), I still find it fundamentally a well-designed games even with its flaws.
I can also technically count Pokemon Uranium here as well since it is indeed a free download. It's by far the best fan game I've seen for Pokemon. It has a significantly better and darker story than most of the mainline games, interesting fake mons, a solid region design... I'd say it's not as good as like Black and White 2 and Platinum, but other than that, it's honestly the best mainline-sort of pokemon game there is.