Back in 2006, while still in highschool, I wrote a level editor for Duke Nukem 2. I've never shared it publicly before, as the code is pretty crappy and I have no plans of developing it any further. But maybe it'll be of use to someone, so I decided to release it:
https://github.com/lethal-guitar/Duke2Edit
I'm happy to answer questions if anyone is interested in picking this up and working more on it!