Cheat Codes
dnkroz or dncornholio: god mode
dnstuff: full ammo and weapons
dnitems: full inventory and all keys
dncashman: Duke throws cash when you hit spacebar
dnscotty[episode][level]: warps you to desired level
dnrate: shows frame rate
skill[skill number]: changes difficulty level

Cheat codes that are useless:
dncosmo: tells you to buy Apogee's Cosmo game.
dnallen: tells you to buy Apogee's Major Stryker game.
dnbeta: says "Pirates suck"
dnview: changes to chase view (not totally useless, but F7 does the same thing).