One of my favorite games is an Open Source game called Dungeon Crawl Stone Soup. It's a Dungeon crawler/rogue game with huge amount of options. The graphic is simple which take you back to the early 80's (see nostalgia). Despite I have Steam with over 400 games, Stone Soup is far the most enjoyable game I have played to date. Only the Forgotten Realms (D&D) games to C64 can compete with it in my opinion.
It's also free (as in free beer) so grab it in the Application Store!!! And better yet it runs on low-end computers as well.