The Mud Emporium was a website I ran for several years dedicated to archiving MUD areas and MUD code. I eventually shut it down, but I did save all of the content.
All of the content for the Mud Emporium is now on Github, seperated out into two repositories.
The area repositories contains hundreds of areas for many different code bases, ROM, Diku, ect.
The code repositories is full of code snippets to add classes, skills, spells, NPC classes, and many more behaviors to a MUD.