Duke Nukem 3D: Megaton Edition has just added a level editor and integration with Steam Workshop to allow creating and sharing player-made maps.
The developers have put together a handy Steam Guide
to help get started with using the editor and sharing to the Duke Nukem Workshop.
It looks like the community has already been busy helping test the new integration, and there are over 60 new maps already available in the Duke Nukem Workshop
If you haven't yet picked up Duke Nukem 3D: Megaton Edition, you can get it for 75% off
when you buy it before December 16th.