To use these codes, just type them in at the console.
Press the ~ key to get to the console

GOD               God Mode
NOCLIP            No Clipping Mode
IMPULSE 9        All Weapons and Keys
IMPULSE 255    Quad Damage
FLY               Fly Mode
MAP E#M#      Level Warp
NOTARGET   Nothing sees you untill you shoot.
KILL    Suicide
IMPULSE -1   Everything but the weapons.
IMPULSE 11   Jewels.
GIVE #    Gun number.

GIVE C #   Charge (# of charge wanted.)
GIVE N #   Nails (# of nails wanted)
GIVE R #   Rockets (# of rockets wanted).
SV_GRAVITY   Change Gravity.
GIVE HEALTH #   Health (# up to 999).
 

-+-+-+- Quake 2 -+-+-+-

Press ~ before you type these:

god            - Enables god mode
g_unlimited_ammo 1   - All ammo gets stuck at 999
noclip         - Turn off clipping
notarget      - Invisibility
map end       - Jump to the level end boss

Also, you can type 'give <name of weapon/ammo> and # of ammo' ex: for 100 shotguns, type 'give shotgun 100'
 

-+-+-+- Quarantine -+-+-+-

Start the game with these paramaters:

-cheater -infinite -nocar -invince

In game: press Alt-Ctrl-F7 before anyone hits you or you hit them and you will be invincible, no cars on the road, and infinite weapons.
 

-+-+-+- Quest for Glory 2 -+-+-+-

During the game, type SUCK BLUE FROG to enable the cheats.

Now hold down ALT and type these codes:

B  - Edit account and communication
K  - Edit a skill
H  - Edit all skills
I  - Edit items
T  - Teleporte to a special place
S  - List of all sprites that are currently on the screen
E  - Ego stats
 

-+-+-+- Quiver -+-+-+-

WUSS   - God mode
NUKE   - Kills all bad guys
MMAP   - On automap (TAB) will display entire map
PANSY   - All weapons, all ammo
BUTTHEAD  - All keys, all objectives
STUN   - Temporarily stuns bad guys

WARP   - Jumps to a specific level