As the name suggests, its goal is to provide a complete open source system for authoring and playing interactive fiction games. Due to its modular design, the front-end can be easily changed, thus allowing the creation of text-based user interfaces, while still supporting graphical clients (e.g. using X ).
Frotz - a portable Z-Machine interpreter
The Z-Files - Lists of IF games
For those with more, um, "salacious" interests...
Decompilers for numerous "interactive fiction" game engines, all written in Haskell .
Nethack is a classic text-based graphical game involving exploring a dungeon in search of the "Amulet of Yendor". Very, very, very difficult. And quite hilarious!
A version of the classic space trader game, written in Section 2 Lisp.
A multi player space game, played via close interaction with "PostgreSQL". Play using SQL queries, or write scripts to implement your own favorite interface.