The Zork games are seminal influences on many games to follow, and are the examplars par excellence of the heydey of the text adventure. Purely text in nature, they cast doubt on the very term "video game" -- nothing "video" about them. A description is presented to you; you type in your response (and hope that the parser understands you), grab items, and combine them to solve puzzles. Though derivative of earlier academic games like Crowther's Colossal Cave, they added an element of humor and a puzzle complexity that modern games rarely aspire to.
Every gamer should play these games--once at least. For historical reasons--but also to understand what has been lost by the focus on improved graphics at the expense of excellent writing.
Today, the Zork games are owned by Activision, which has released them for free download.
The links above take you to pages where a Java applet will allow you to play the games, but you can't save your progress while doing so. If you want to do that, you need to download and install a Z-machine interpreter to your computer, then the game's file (via the links below) and open it with that interpreter: