I don't know of a better turn-based computer game. Dungeon Crawl Stone Soup is the best there is and has been so for a long time. Forget the hoopla about commercial games or fancy graphics. DCSS has everything one will ever want in a game and the replay value is excellent, in part because the game receives active development and constantly is getting new features. Dungeon Crawl Stone Soup fully supports Linux, Windows and Mac. The system requirements are modest.
For those who feel that the game is too tough, see http://techlorebyigor.blogspot.com/search/label/dungeon%20crawl%20cheat for two methods of making the game easier.