I was directed to this game by a retweet by @haggismaths. It’s a roguelike (text-based explorey role-playing adventure game) which takes place on the hyperbolic plane. It’s a lot of fun. It’s hard to get your head round the fact that there’s a lot more stuff in between two lines in hyperbolic space than in Euclidean space, so it’s very hard to find your way back somewhere after it disappears over the horizon.

You can download a windows executable, or source code which will compile on Linux, at http://www.roguetemple.com/z/hyper.php