From a dos prompt in your Quake3 directory, type:
quake3.exe +seta g_spVideos "\tier1\1\tier2\2\tier3\3\tier4\4\tier5\5\tier6\6\tier7\7\tier8\8"

Unlock All Levels at Skill 1:

Bring down console and type: /iamacheater

Unlock All Levels at Skill 100:

Bring down console and type: /iamamonkey

Hidden Sarge Skin:

Bring down console and type: /model sarge/krusade

In order to use the following cheats in multiplayer mode, the server must have cheats enabled (ie: /devmap +map [name])

God Mode:

Bring down console and type: /god

All Weapons:

Bring down console and type: /give all

You can also use the following codes with the /give command:

allammo
armor
battle suit
bfg10k
flight
gauntlet
grappling hook
grenade launcher
haste
health
invisibility
lightning gun
machinegun
medkit
personal teleporter
plasma gun
quad damage
railgun
regeneration
rocket launcher
shotgun

Cheat Mode for Q3DemoTest:

Load Q3DemoTest, open the console, and type /spdevmap [map name]

Use the one of the following values for:

q3dm1
q3dm7
q3dm17
q3tourney2

The game will load in single-player mode with the current settings of the bot variables, and cheat mode enabled. You can then use the cheats listed above.