Twine - Twine is an open-source tool for telling interactive, nonlinear stories.
Mudlet - Mudlet is a freshly-minted MUD client, designed to take mudding to a new level.
RenPy - Ren'Py is a free and cross-platform visual novel engine that helps you use words, pictures, and...
TinTin++ - TinTin++, aka tt++, is a free MUD client for Mac OS X, Linux, and Windows.
Quest - Quest lets you create sophisticated text-based games, without having to program.
CMUD - Are you tired of the constant grind of the graphical MMO games?