During play, press F10 and enter the following. Some of these will not work with patched versions of the game.

godgames        God Mode 
winblows        All Weapons and Ammo 
skeletonkey     Get Skeleton Key 
moreammo        More Ammo 
goremode        Toggles Gratuitous Dismemberment 
buringstake     Get Flaming Tip Arrows 
thunderstorm    Rains Outside 
snowstorm       Snows Outside 
bighead         Big Head Mode 
healme          Restores Health 
silver          Get 500 Silver Bullets 
aqua            Get 500 Aqua Vampire Bullets 
mercury         Get 500 Mercury Bullets 
oldhat          Different Hat
goldmode        Secret Message
freezer         Toggles Enemy Freezing 
ifarted         Gas Mask 
headofhorrors   Enable bigboom Code below 
bigboom         Instant Kill 
recharge        Renew Battery 
dumbogun        Elephant gun
shotgunshell    Shotgun and/or 500 shells
oldhat          Stranger wears old hat
baronsaturday   Summons Baron Samedi
gimmecrap       Tommy gun

Hints

Thanks to Revolution readers Patrick "The Stranger" Kofler, Ashton Krupa, WLP, Ali J, mike mcnair and Corey Grimm!