Ancient Domains of Mystery

Ancient Domains of Mystery is a massive, fiendishly difficult Rogue-like fantasy freeware game, rendered in ASCII, available for all three major OSes, and created entirely by one juggernaut of a man. It is profoundly unfriendly to new players (who can resort to fan sites for help); there is no tutorial, just a huge manual to read. However, once you get the swing of things, this game can be a great deal of fun and is very addictive. The game's mechanics are largely based on Dungeons and Dragons, with to-hit modifiers, damage dice, and weapon skills, and dedicated players can crunch the numbers to optimize their butt-kicking, or you can just grab the first artifact weapon you see and go crazy. The depth of the game is staggering; there are astrological signs which affect the character, dozens of magic spells, potion crafting, magic herb growth based on the Game of Life, and hideous mutations sustained by the player character as s/he struggles against (or for!) the invading forces of chaos.
ADOM will not appeal to all gamers; it caters to stubborn, devoted fantasy buffs. However, those who will take the time to learn to control the complex world will grow to love it.