Nethack

A top down, text art dungeon crawling game in the tradition of 'Rogue', Nethack was developed in 1987, and was intended to be a follow up to the RPG 'Hack'.  Nethack was so named because it was a development of a new 'Hack' game created through co-operation of many programers over the still young 'internet'.  Net hack is still activly developed and expanded by a team of developers.  The length of development has cause a great deal of complexity to be written into the mechanics and interactions.  To this day, there is a known buglist, but it contains mainly esoteric issues, and issues of compatability with select operating systems it has encounterd over it's 20 year lifespan.  As an opensource game, there are derivitives abound, as well as patches to expand on the gameplay, a very common change is to add graphics tiles to the game, instead of somewhat abstracted text art system.  There is also an isometric 3d client which runs the base game.