Also, a level editor
As a kid, I played hundreds of hours of Duke Nukem 3D. I even went so far as to go to local tech stores and find "level pack" CDs so I could play more maps than the game could possibly offer, and made a few of my own in my spare time.
Thankfully the Steam version of Duke 3D (the Megaton Edition) has just added Steam Workshop support and a level editor. Basically, the editor allows you to create extra maps for the community and the Workshop lets you share them -- pretty simple stuff.