Special level (NetHack: The Next Generation)
The Forge is the home of the Blacksmith, who can forge selected artifact weapons for you.
The School, located in Gehennom, contains teachers, pupils, and a principal.
The Computer Level
The Computer Level contains several hostile computers. In NHTNG, this is also the only place where certain (mostly nonfunctional) objects will appear.
In SLASHTHEM the Dragon Lord appears in the Wyrm Caves.
The Hitchhiker's Level
The Hitchhiker's Level is a tribute to Douglas Adams and his Hitchhiker's Guide to the Galaxy series. It contains a number of monsters that represent characters from the series or otherwise allude to it.
Spoiler by Sebastian Klein