I'm surprised hardly anyone else has mentioned Wesnoth yet. I last played it over a decade ago, so I'm not sure if it's still as well-known now as it used to be, but it's one of the highest-quality open-source games out there.

It's a turn-based fantasy strategy game that feels like a combination of Advance Wars, Fire Emblem, and D&D. And I suppose it was a lot more unique back in the 2000's when there weren't a dozen indie games out there that fit the same description.