There was a time when "free-to-play" was a dirty term in the games industry. Actually, there are still terrible, exploitative free-to-play games on the market—more every day, in fact.
But occasionally—occasionally—we get something...miraculous. We get a free-to-play game that doesn't try to con players out of money or make the design intentionally boring in order to make those purchasable unlocks more exciting.
Here, you'll find a list of games so good the developers could've charged money (or, in some cases, did charge money) before going free-to-play. These aren't just good free-to-play games, they're good games, full stop.
If you're looking for more bite-sized gaming snacks, check out PCWorld's guide to 16 free, fun browser-based games. They're not as meaty as these titles, but they're still fun.