Wander

Wander, by Peter Langston, is an interactive fiction engine which actually predates the "Original Adventure" (Colossal Cave), going back to 1974. The author continued to develop it into the 1980s, but it was still regarded by some computer historians (who apparently never bothered to try to contact the still-living author) as "lost" until various versions turned up. Its "world files", containing all the data of a particular adventure, have a .wrld extension.

Links

 * Blog post about it, linking to various downloadable versions
 * Another blog post