3DRealms still sells the game off their website at 3drealms.com (in fact, they recently did an updated print run, believe it or not). If you don't have it, this is the path of least resistance to a legitimate copy of the game. eBay is another option. Make sure you get the "Atomic Edition" if you buy a PC disc.
TCP/IP networking and an OSX version of the level editor are included, but
- The 12-16-2004 version now allows for taking screenshots
- OS X 10.2 or later
- Duke Nukem (retail purchased) CD