Now that Duke's out again, who has any great user maps they remember from yester-year to recommend to new players? I'd say the definite starting point is Duke4.net's 5000+ (yes, five thousand) map download, found here, alongside the excellent Roch campaign:
http://www.duke4.net/download.php?list.5
My favourite single player user map was always Phantasm (with an "s"), which is in that package. It's bizarre, but I was hooked the minute I saw that behind-the-pillar trick in one of the first rooms, which you'll find in a few maps if you poke around!
What's your favourite?

I still got the 500 levels expansion pack... but I don't remember any good map in it.

There are some good map paks here:
http://dukenukem3dtc.tripod.com/id131.html
i would start with
Aqua Map Pak
Duke Nukem is 007 TC
Duke Nukem Unleased
Fakirs Maps 1 to 3
MP's Crucial Conflict
Red Series
Also DNR and MSDN are very good map sites:
http://www.dukerepository.com/
http://msdn.planetduke.gamespy.com/

Don't forget Penthouse Paradise!
http://rezcycle.livejournal.com/9610.html
Warning, slightly NSFW work images (since the map is slightly NSFW. Well, you shouldn't be playing Duke at work anyway. Get back to work, slacker!)