Spotlight: Interactive Fiction
Interactive Fiction, or 'text adventures' is one of the oldest adventure genres around. The first interactive fiction game, called simply Adventure, dates back to 1975.
One of the most well known series is Zork, developed by one of the most popular interactive fiction companies of all time: Infocom. Other popular developers were Magnetic Scrolls (The Pawn, Corruption) and Legend Enternainment (Gateway, Spellcasting 101).
Almost no one sells text adventures anymore (there are a few exceptions), but a thriving communty of amateur developers still exists.
The best place to start looking for interactive fiction is without a doubt The Interactive Fiction Database, a huge database of both old games and recent ones.
Back in the day, text adventures were easy to play: insert the disk (or cassette, for the really old ones) run the executable and play. Now the vast majority of amateour interactive fiction games need an interpreter. I'm not writing a guide here, but I strongly advice to read this Beginner's Guide to Playing Interactive Fiction.