Open up the console using Ctrl+A+~ and type iddqd. You'll get a secret message.

Doom 3 Custom Flashlight Textures
To make a custom texture for your flashlight in Doom 3, create a .pk4 file or copy one from your Doom 3 directory and clear it out. You can use a compression program such as WinZip to open and compile .pk4 files.
Once you have an empty .pk4 file to work with, create a folder named 'lights' (without quotes). Grab the image you want to use and convert it to a .tga file (256 X 256 pixels is a good size). When you use the image it will be reversed, so mirror your image before saving it. Name the image flashlight6.tga, place it in the lights folder and make sure that folder goes in your .pk4 file. Then, rename the .pk4 file so it is the highest number in your Doom 3 directory. For example, if you have a pak005.pk4 file already, you will have to name your new flashlight texture file pak006.pk4 for it to take effect.
Once you have renamed the file, place it in your base directory and start the game.

Have you ever noticed what happens when you kill an important character and he comes back to life at the next scene? It only happens to the main characters, so take advantage of that.
For example: In the lvl 'Mars City Underground' just before you get outside on mars, there is a security guard standing there with a pistol, I shot him in the head and shot him a couple more times to get rid of the body, then I entered the inner airlock door and continued my mission.
When demons start coming into the UAC facility, you make your way back to the same point. I asked myself 'Where is the guard I shot earlier?' His body was never possessed by a hell soul because it wasn't there.. think about that advantage you can have..

Hi fellow Pc/X-box players. I am here to tell you 'the best' weapons to use against some of Doom 3's finest enemies.
Zombie type: Zombie type monsters didn't exactly come from hell, but they were resurrected by the evil souls of hell. All these monsters need is 3-6 pistol shots, or if you like getting nasty then 1 shotgun blast up close should knock them down cold, leaving nothing! Although some zombies like the chainsaw zombie are hard to kill, it would hurt to bring out the machine gun.
Trites and Ticks: Well well well, aren't we all afraid of spiders? No way! These guys are the weakest of the weak, 2-5 pistol rounds should do'em good.
Imps: These guys first scare you when you first meet them, but when you get used to it they become just a little ant to you. 1 shotgun blast up close and this guy burns his way back to hell.
Pinky Demon: These half machine half bull creatures freak me out. They will run at you like a bull until it's caught you in a corner and chewed your head off. 2-3 shotgun blasts up close (I wouldn't wanna get near that thing!) should take it down for good.
Lost Soul: These are the 2nd weakest but the fasted enemies in the game. Just because I said they're weak is doesn't mean your safe, they travel in packs and make a human like cry as they fly toward you. Using the chain gun from far away should be quite easy taking them out.
Hell knight: One of the strongest demons in the game. These guys like to throw green flame at you, or if you get to close he picks you up and throws you across the room. Using a rocket Launcher or a plasma gun from a distance, you can easily make a few hits and runs with the rocket launcher. Just don't get close or..its too gory too explain.
Cherub: An insect like creature with the face of a baby and the body of a bee. They like to travel in small packs. Take a plasma gun from far away and take these things down before they get into your dreams! Or to make it fun, you can take the machine gun or chain gun and spray the place to get rid of them slowly.
Revenant: I guess keeping this guy 6 feet under the ground in a coffin wasn't the brightest plan, now that you see he escaped, found a couple of rocket launchers and mounted them on his shoulders. A chain gun, plasma gun, or rocket launcher do these guys really well.
Cacodemon: Big, floating, slow and easy too kill because it's so slow. The rocket launcher is the perfect weapon to blow a hole in these things.
Mancubus: The strongest non-boss enemy in Doom 3. Because its so heavy and strong, its super slow. Mounted on each arm is a rocket launcher that fires more than 1 rocket at a time. Rocket launcher, plasma gun or a BGF 9k should work on these big demons, but it would take a lot of ammo to finish him off. Keep moving while firing and you're safe.
Boss's: Its up to you how you choose to kill them..

In the final level, Excavation Site, after going down the elevator into Hell, stick to the left wall. Make sure to stay to the left, or the final cutscene will trigger. You should come to a dead end with candles barely lighting a brick with the id logo in the center. Click this brick and a wall will open to your left. Walk inside the room to find the id Software PDA containing 'Thank You' emails from the staff.

remember, sometimes people want to enjoy a game without the lag.try turning off the shadows. its just one more texture that your computer has to deal with.
maybe some of the ambiance is lost but for the most part it will not lag as much. also there are some great mods out there that are easy to install and the great thing about them is they give your walk/run speed, ammo and all your health an added boost.
check out doom3filefront.com they have some pretty cool mods from adding more gore to speeding up your game.

Open up the console by pressing CTRL + ALT + ~. Type in devmap game/(map name), where you substitute (map name) for a map below.
marscity1.map - Mars City 1
mcunderground - Mars City Underground
marscity2.map - Mars City 2
admin.map - Administration
alpha1.map - Alpha Labs Sector 1
alpha2.map - Alpha Labs Sector 2
alpha3.map - Alpha Labs Sector 3
alpha4.map - Alpha Labs Sector 4
enpro.map - EnPro Plant
commout.map - Communications Transfer
communications.map - Communications
recycling1.map - Monorail Skybridge
recycling2.map - Recycling Sector 2
monorail.map - Monorail
delta1.map - Delta Labs Level 1
delta2a.map - Delta Labs Level 2A
delta2b.map - Delta Labs Level 2B
delta3.map - Delta Labs Level 3
delta4.map - Delta Labs Level 4
hell.map - Hell
delta5.map - Delta Complex
cpu1.map - CPU Complex
cpuboss.map - Central Processing
site3.map - Site 3
caverns1.map - Caverns-Area 1
caverns2.map - Caverns-Area 2
hellhole.map Primary Excavation

You will need to edit the DoomConfig.cfg file, located in the /Data/BASE folder. Make sure you backup the file before making any changes. Open the file with Notepad or other text editor. Look for the 'g_nightmare' entry. If you haven't finished the game yet, you will see the number 0. Change it to 1, save, and nightmare mode will be unlocked.

When you start the game, go through out the game and find the kitchen before 'all hell breaks loose' and in the corner of the kitchen you'll find an arcade game..On the screen is the face of the original Doom character..And he also makes the same faces when you punch and kill the turkey.. Get a highscore of 25,000 or higher to get a mail on your PDA.

In various parts of the game you will come across weapon lockers that store Weapons, Ammo, Health, Armor, and tons more. The number on the left is the locker number, while the number on the right is the combination required to open it. Here are all the combinations for the lockers.
#001 - 396
#003 - 483
#009 - 752
#013 - 586
#017 - 347
#023 - 531
#038 - 409
#039 - 102
#047 - 123
#048 - 123
#049 - 123
#054 - 246
#054 - 142
#063 - 972
#064 - 651
#078 - 364
#079 - 364
#103 - 259
#104 - 579
#112 - 538
#114 - 715
#116 - 972
#116 - 624
#117 - 624
#213 - 371
#21D - 298
#317 - 841
#386 - 836
#387 - 836
#452 - 571
#666 - 372
#669 - 468
Martian Buddies 1 and 2 - 0508

Unknown console commands
The effect of these console commands are not documented, but some of them are pretty obvious since they are similar to other id or even other shooter commands.
_use
_moveRight
_moveLeft
_moveDown
_forward
_back
_right
_left
_lookDown
_lookUp
_attack
_moveUp
_speed
_strafe
_button7
_button6
_button5
_button4
_button3
_button2
_button1
_button0
aasStats
addarrow
addline
blinkline
bindlist
bind
centerview
compressDemo
connect
clearlights
conDump
clear
cvar_restart
cvarlist
combineCubeImages
crash
collisionModelInfo
dirtyfeet
dir
demoShot
disconnect
dmap
echo
envshot
exec
exportmodels
editSounds
editLight
editor
error
fillrate
flashlight
focusSoundsEditor
give
game_memory
in_restart
kill
killRagdolls
killMoveables
killMonsters
keepTestModel
listModels
listSkins
listMaterials
listImages
listModes
listLightDefs
listEntityDefs
listMonsters
listEntities
listThreads
listSounds
listAnims
listCollisionModels
listlines
listVertexCache
map
modulateLights
noclip
notarget
nextanim
overlayGUI
path
poplight
parsewait
printShader
printMode
playDemo
playCmdDemo
playerModel
prevanim
reload
reloadSounds
reloadSoundShaders
reloadFx
reloadanims
reexportmodels
removeline
runReach
runAAS
recordCmdDemo
recordDemo
recordCmdDemo
recordDemo
renderbumpFlat
renderbump
roq
reloadGuis
reset
regenerateWorld
reloadImages
reloadShaders
reloadModels
script
reloadEntityDefs
s_musictrack
s_cacheInvalidate
s_cacheInfo
s_restart
seta
set
spawn
setviewpos
setStepGamma
setu
sets
sizeDown
sizeUp
screenshot
spawnServer
stopRecording
say
testTrace
testImage
testShader
testBump
testLight
touchFx
touchGui
touchFile
toggle
testSIMD
touchMaterial
testFx
touchSound
listSoundShaders
touchParticle
testDamage
testblend
testanim
testParticle
testModel
testWipe
testGUI
timeDemo
testmap
touchSkin
touchModel
unbind
unbindall
vid_restart
vstr
writeConfig
writePrecache
weapon
weapnext
where
weapprev
weaponSplat

Press Control+Alt+Tilde(~) To open the console, or just tilde if the com_allowconsole 1 cheat is typed in. The 'god' and 'notarget' codes are recommended before spawn a monster.
Spawns a Wraith demon - spawn monster_demon_wraith
Spawns a Fat2 zombie holding a wrench - spawn monster_zombie_fat_wrench
Spawns a Fat zombie with his intestines showing - spawn monster_zombie_fat2
Spawns a Jumpsuit zombie - spawn monster_zombie_jumpsuit
Spawns a bald 'Yellow Jacket' zombie - spawn monster_zombie_maint_bald
Spawns a No Jaw 'Yellow Jacket' zombie with a flashlight - spawn monster_zombie_maint_flashlight
Spawns a 'Yellow Jacket' zombie with no jaw - spawn monster_zombie_maint_nojaw
Spawns a Skinny 'Yellow Jacket' zombie - spawn monster_zombie_maint_skinny
Spawns a No Jaw 'Yellow Jacket' zombie with a wrench - spawn monster_zombie_maint_wrench
Spawns a completely black 'Yellow Jacket' zombie - spawn monster_zombie_maint2
Spawns a Chainsaw zombie - spawn monster_zombie_sawyer
Spawns a skinny zombie - spawn monster_zombie_skinny
Spawns a Suit zombie with a bloody mouth - spawn monster_zombie_suit_bloodymouth
Spawns a Suit zombie with no head - spawn monster_zombie_suit_neckstump
Spawns a Skinny Suit zombie - spawn monster_zombie_suit_skinny
Spawns a Bald zombie similar to the one in the infirmary - spawn monster_zombie_tshirt_bald
Spawns a zombie almost exactly like the one in the infirmary - spawn monster_zombie_tshirt_blown

Here are a few spawning zombies cheats (They will spawn in front of you, you have ot be in a kind of open space, and space before typing spawn)
-Fat Zombie: spawn monster_zombie_maint_fat2
-Zombie: spawn monster_zombie_maint (or you can type maint2 insted of maint for different zombie)
-Zombie w/ flashlight:
spawn monster_zombie_maint_flashlight
- zombie w/ tshirt:
spawn monster_zombie_maint tshirt