Not really sure about any free games that I would personally define as good.
Most games I play are paid, however, people have their preferences.
If you are into first person shooters, you can play CrossFire, or Combat Arms.
However, those games are more of a pay2win game.
There is this one game that I always get back to every year, however, I end up quitting after a couple of weeks.
That would be Maplestory. I guess it brings back childhood memories when I was a kid.
There is another game that seems to be making a comeback and that would be Runescape. I don't play that, but it is worth mentioning.
If you're talking about paid games, you have more of a variety of options that can fulfill your month.
There is also the option of cracking the games, so that you don't have to pay for it.
I personally play Terraria right now and there is a cracked version for it. (I play the paid version)