This guide is inspired by the official list of cheat codes, available at:
http://web.archive.org/web/20141012204122/http://www.3drealms.com/cheat/duke3d.html

I've created this guide to make it easy to find cheat codes from within the Steam Community overlay (Shift+Tab screen). Although it is mostly the same as the official list, I've added and corrected some bits (marked in italic).

To use a cheat code, just type it while in-game. Some cheat codes can be disabled by typing them again. Cheats are only active during the current play session, quitting and restarting the game will clear all cheats.

Using a cheat code unlocks the "Shame" achievement, but only if you are using the public beta version of Duke Nukem 3D.

There is also an official walkthrough (listing all secrets) available at:
http://web.archive.org/web/20141011005019/http://www.3drealms.com/duke3d/walkthrough/

DNCORNHOLIO - God Mode (And Unlimited Jetpack).
DNKROZ - Same as Cornholio, just quicker to type.
DNVIEW - Same as the F7 key in the game. (Also prints the message "You're too good to be cheating")
DNHYPER - Puts you in Steroids Mode.
DNINVENTORY - Gives all inventory items and armor. (Not listed in the official guide.)
DNITEMS - Gives all inventory items and keys.
DNWEAPONS - Gives all weapons and ammo.
DNKEYS - Gives all the keys.
DNSTUFF - Gives all weapons, keys, inventory items.
DNSCOTTYxyz - Level Warp. x is the Episode Number, yz is the level number. (Ex: dnscotty103 for Episode 1 Level 3, or dnscotty211 for Episode 2 Level 11)
DNSKILLx - Changes the difficulty mode to x. (x can be 0 to 3) (0 means no monsters, 1 is a hidden very-easy mode, 2 is Piece of Cake, 3 is Let's Rock, 4 is Come Get Some, 5 is Damn I'm Good; values 6-9 behave just like 5, but don't disable cheats)
DNUNLOCK - Opens all doors and unlocks all locked doors.
DNMONSTERS - Turns off (or on) the monsters in the game. (They disappear as soon as they get active.)
DNCLIP - Walk Through Walls (Tip: open the map to better see where you are.)

Warning: Being in God Mode can crash the game at certain points, depending on what you are doing. Being in God mode when you are squashed by rotating gears is one example.

Warning 2: The DNCLIP code can be very dangerous. It can cause crashes at any time. If you have a problem, and have used DNCLIP, restart, and do not use DNCLIP, and see if it happens. This was the main reason we didn't have a DNCLIP in v1.0.

DNSHOWMAP - Shows the entire map.
DNCASHMAN - Spews money everywhere when you press space the use key.
DNCOORDS - Shows extremely detailed map coordinate/level information.
DNDEBUG - Shows some debug information.
DNRATE - Shows your frame rate onscreen. You must be in full screen mode for this to be anywhere near accurate.
DNBETA - Prints the message "Pirates Suck!"
DNALLEN - Prints the message "Buy Major Stryker"
DNTODD - Prints the message "Register Cosmo Today!"