For  all cheats you have to go to the console (hit the ~ button),
type in the code and hit return...

Cheat code:             Effect:
god                     Godmode ON/OFF
noclip                  Noclipping ON/OFF
notarget                Enemys don't recognice you ON/OFF
skill #                 Changes skill level (0-3)
map X                   Changes level on client (X-Level Name)
changelevel X           Changes level on server (X-Level Name)

LEVEL NAMES

1                       Blackmarsh
demo2                   Barbican
demo3                   The Mill
village1                King's Court
village2                Inner Courtyard
village3                Stables
village4                Palace Entrance
village5                The Forgotten Chapel
rider1a                 Famine's Domain 
meso2                   Plaza of the Sun
meso1                   The Palace of Columns
meso3                   Square of the Stream
meso4                   Tomb of the High Priest
meso5                   Obelisk of the Moon
meso6                   Court of 1000 Warriors
meso8                   Bridge of Stars
meso9                   Well of Souls
egypt1                  Temple of Horus
egypt2                  Ancient Temple of Nefertum
egypt3                  Temple of Nefertum
egypt4                  Palace of the Pharaoh
egypt5                  Pyramid of Anubis
egypt6                  Temple of Light
egypt7                  Shrine of Naos
rider2c                 Pestilence's Lair
romeric1                The Hall of Heroes
romeric2                Gardens of Athena
romeric3                Forum of Zeus
romeric4                Baths of Demetrius
romeric5                Temple of Mars
romeric6                Coliseum of War
romeric7                Reflecting Pool
cath                    Cathedral
tower                   Tower of the Dark Mage
castle4                 The Underhalls
castle5                 Eidolon's Ordeal
eidolon                 Eidolon's Lair
ravdm1                  Atrium of Immolation
ravdm2                  Total Carnage
ravdm3                  Reckless Abandon
ravdm4                  Temple of RA
ravdm5                  Tom FooleryDemo

playerclass #           Changes player class (1-4)
restart                 Restarts the level
name X                  Changes your name (X-New Name) 
give h X                Gives health (X-Amount of health. Maximum 999) 
give 2                  Gives weapon 2
give 3                  Gives weapon 3
give 4                  Gives weapon 4
impulse 9               All Weapons/Mana
impulse 14              Gives you a Sheep
impulse 23              Torch
impulse 43              All Weapons/Mana/Items
impulse 44              Throws a Item
impulse 10              Change Weapon
impulse 13              Lift Object
impulse 100             Uses the Torch
impulse 101             Uses the Quartz Flask
impulse 102             Uses the Mystic Urn
impulse 103             Uses the Krater
impulse 104             Uses the Chaos Device
impulse 105             Uses the Tome of Tower
impulse 106             Uses the Summon Stone
impulse 107             Uses the Invisibility
impulse 108             Uses the Glyph
impulse 109             Uses the Boots
impulse 110             Uses the Repulsion
impulse 111             Uses the No Peep
impulse 112             Uses the Ring Of Flight
impulse 113             Uses the Force Cube
impulse 114             Uses the Icon Defn
chase_active 1          Chase Mode (Camera) 
r_fullbright x          Lights on
status                  Information about the current situation
crosshair x             Crosshair ON/OFF
help                    Help menu
clear                   Clear console window
echo                    Text will be displayed on the screen
screenshot              PCX dump
save                    Save as
load                    Load
kill                    Player dies and restarts the level
timerefresh             Frames per second
showturtle x            If the frame rate is blow 10 a turtle
                        will be displayed
version                 Version number
sv_friction #           Changes friction (4)
sv_gravity #            Changes gravity (800)
sv_maxspeed #           Max speed (800)
r_drawentities x        All objects ON/OFF
r_drawflat x            All textures ON/OFF
r_waterwarp x           Underwater effects
fov #                   Change view (90)
d_mipscale #            Change mip scale (1)
nosound x               Sound ON/OFF
vid_describemode #<     Informations about mode #
vid_describemodes       Shows all graphic modes
vid_describecurrentmode Informationen about current graphic mode
vid_mode #              Changes graphic mode
vid_nopageflip x        Page flipping ON/OFF
vid_nummodes            # of modes
vid_testmode #          5 sec graphic mode test
vid_wait                Sets wait mode
demos                   Plays the built-in demos
playdemo                Plays an own demo
record                  Records a demo
stop                    Stops the recording of a demo
cd eject                Ejects CD
cd loop                 Endless loop of track #
cd off                  CD OFF
cd on                   CD ON
cd play                 Plays track #
cd remap                Reconfigures the numbers of the tracks
cd reset                Reloads the CD
cd stop                 Stops CD
cd resume               Resume CD
menu_keys               Open keys menu
menu_save               Open save menu
menu_net                Open multiplayer menu
menu_main               Open main menu
menu_options            Open options
menu_load               Open load menu
say                     Message to all players
say_team                Message to team players
color                   Changes color of clothes
connect (server)        Connects with server, also COM #
maxplayers              Max # of players
hostname                Change host name
net_stats               Network stats
slist                   List of all server
ping                    Speed of network
soundinfo               Sound info
stopsounds x            All sounds ON/OFF
volume #                Sound volume
bgmvolume #             Music volume
bgmbuffer #             Sound buffer
snd_show x              Sound debugger
cl_rollspeed #          Strafe speed
cl_rollangle #          Sidestep angle
v_kicktime #            Kicktime
v_kickroll #            Kicktime strength
v_kickpitch #           Kicktime strength with hit
r_drawentities x        Show items
r_drawviewmodel x       Show weapons?
r_draworder x           Transparent walls
r_dspeeds               Stats
r_maxedges #            Max # edges
r_polymodelstats        Stats
r_speeds                Stats
r_timegraph             Show graph
sv_nostep x             Player can't walk over steps
sv_maxvelocity #        Max ammo speed
d_mipscale x            Fuzzy
d_subdiv16 x            Perspective calculated only all 16 pix
m_pitch #               Up-down sensitivity
m_yaw #                 Turn around sensitivity
sizeup                  Enlarge screen
sizedown                Small screen
bf                      Redraw
centerview              Center view
viewsize #              Change view size
showram x               RAM symbol ON/OFF
showpause x             Pause symbol ON/OFF
lookspring x            Center view after mouse lock ON/OFF
bind                    Set key to a certain command,
                        e.g. BIND W "IMPULSE 9"
alias                   Combine commands
sensitivity #           Change mouse sensitivity
scr_conspeed #          Change console speed
scr_ofsx                Current x position
scr_ofsy                Current y position
scr_ofsz                Current z position
pause                   Pause the game
togglemenu              Console ON/OFF
unbindall               Erases all combinations
profile                 Stats
edictcount              Stats
entities                Stats