One of the biggest games you've never heard of is celebrating its 20th birthday this month. On July 28, 1987, Mike Stephenson released NetHack, a text-based RPG that would become one of the most influential open-source computer games ever coded. While the game itself wasn't new or unique, (it was based off of previous games like Rogue and Hack, that spawned the genre of "Roguelikes"), its development and license makes it an influential part of the annals of gaming history.
NetHack was one of the first games to adopt the open-source General Public License, a software license that lets any user download, distribute, or modify its code, as long as that code remains free and available. Because of this, NetHack has spawned dozens of variants and spin-offs. Amazingly, NetHack remains in development to this day, with Stephenson and his friends still working on it. The current version of NetHack, 3.4.3, was released in 2003, and a new version will be released "when it's ready."
NetHack and other Roguelikes will probably seem strange to most modern gamers. Before polygons and sprites, before bump mapping and pixel shading, before Kratos' scowl and Cloud's hair, we had letters, numbers, and symbols. While graphically enhanced versions are available, NetHack originally used (and still uses) ASCII characters to represent everything. Your character (@) and his pet dog (d) or cat (f) must brave the dark corridors (#) of a massive, randomly-generated dungeon and fight the kobolds (k), bats (B), and leprechauns (l) within to retrieve the Amulet of Yendor ("). These Roguelikes might look like a bunch of letters shuffling around a chalkboard, but it was a pretty darn clever way to put visual, dungeon-crawling action into older hardware. Best of all, if you can read this post, you can be almost certain that your computer (or PDA, or cell phone) can run NetHack.
Aspects of Rogue, Hack, and Nethack can still be found in modern games today, in almost every dungeon-crawling RPG out there. Diablo, the archetypical dungeon crawler, uses huge, randomly generated dungeons, hordes of monsters, and hoards of loot in almost the exact same way as NetHack and its kin.