Duke Nukem 3D is without a doubt one of my favorite games of all time. I can't even remember how many times I've re-played the original episodes let alone remember how many different user-made maps or total conversions I've finished. I even became pretty good at making maps after trying it non-stop for 3 years. Never thought that 14 years later I'd regret not making backups of them... All that hard work is now gone forever...
Anyway, I'm pretty sure that anyone that has played Duke3D just won't get enough of it. For that reason I decided to show you 6 websites that'll make you spend many more weeks and months playing this great shooter.
I personally don't like playing the game this way because I don't think it looks better. The High Resolution pack makes Duke3D look like an avarage-looking 'true' 3D game instead of a great-looking 'not-completely-true' 3D game. Don't get me wrong, this add-on is pretty amazing but I prefer the charm of the original instead of this revamped one.
Much more interesting is EDuke32. This great piece of software lets you play the original game on all kinds of OS'es in high resolutions and add tons of extra gameplay improvements if you're used to current FPS standards (f.e. mouse aiming). If you ever want to play Duke Nukem 3D on modern systems without having too many problems EDuke32 is probably a must-have.
MSDN has probably the most complete Duke Nukem 3D map/tc collection around. After you've finished the original game you'll find douzens of extra gameplay hours here.
DNR is another site dedicated to bringing people the finest Duke3D add-ons. Right now they already have more than 600 maps in their database.
Anything, and I really mean anything you need to know about Duke3D modding can be found on RTCM.
If you want to be sure you've found anything that can be found in the Duke3D episodes you should definitely check out this walkthrough written by 3D Realms.