• Battle for Wesnoth is one of my favorite turn-based strategy games.
• Dungeon Crawl Stone Soup makes a whole day disappear when I play it.
• Tales of Maj'Eyal is another, slightly more graphically rich roguelike. I haven't played it too much, but it seems like there's quite a lot of content to it.
• The Ur-Quan Masters... I don't even know how to categorize it. It's tough as nails, but the art style is rich, and the worldbuilding draws you in.

Any others I would have mentioned are already in other comments.