Continuing the Doom theme, here is another conversion that is even madder than the Half Life 2 one.
This is a Doom 2 map wad that gives you the first level of Duke Nuken 3D in Doom! Why? You might very well ask. After all the Duke Nuken 3D engine was as far as I know originally based on the Doom engine anyway, but tweaked and made slightly more advanced. So it does beg the question why bother? Who knows? Some people just have too much spare time on their hands!
Still, it is interesting.
http://www.doomwadstation.com/psyren/duke/