Quake II

Bring up the console (press ~) and enter the following codes:

god	God Mode
notarget	No Target Mode
noclip	No Clipping
give all	All items, weapons, 999 ammo
give jacketarmor	Jacket Armor
give blaster	Blaster
give shotgun	Shotgun
give sshotgun	Super Shotgun
give machinegun	Machine gun
give grenadelauncher	Grenade Launcher
give rocketlauncher	Rocket Launcher
give shells	Shells
give bullets	Bullets
give cells	Cells
give grenades	Grenades
give rockets	Rockets
give slugs	Slugs
give quad damage	Quad Damage
give invulnerability	Invulnerability
give silencer	Silencer
give rebreather	Rebreather
give health	100% health
give weapons	All weapons
give ammo	All ammo
give armor	Body armor

Noticing a pattern? That's because you can type just about anything after 'give' and you'll get it....Try it with any other weapon or item and see!

To skip levels:
Press ~ to access the console, then type map xx where x is the level name. To get level name go to multiplay, host game, and select level name you wish to skip to (the bottom name is the name of the level). For instance, ~ map boss2 will take you to the final level.