The console.





Enable the console by loading the game, and from the menu going to
options->keybord->advanced->enable developer's console.

After that type sv_cheats 1.

Now during gameplay press ~ to get the console [or, on UK keyboards the ¬ button, under Esc and in front of 1 ] and enter a code.


Cvars.
Code Effect
god God mode (Server only)
noclip No clipping mode
impulse 101 All Weapons
notarget Enemies don't target you
maps Map list
budda You receive damage, but won't die
hurtme # Damage player by #
impulse 82 Get a Jeep
hud_quickhelp/text? 1 crosshairs on
exec [file] Run script file
developer # Developer mode (#=0, 1, or 2)
cl_drawhud 0/1 Show or hide HUD
cl_enablehud 0/1 Show or hide HUD
showfps Show or hide FPS
npc_create_aimed Make NPC
npc_create Make NPC
picker picker mode
sv_gravity # Set gravity to number #
sv_waterdist # Set vertical View to #
air_density # Set air density to #
dsp_explosion_effect_duration # Set explosion length
mat_numtexttureunits # Limit texture units to #
prop_debug Bounding box debug?
sv_bounce # Set bounce multiplier
sv_stopspeed # Minimum stopping speed set
setang # Set pitch Yaw to #
sv_friction # Set friction to #
sv_maxvelocity # Set Max velocity


map <mapname> - Use the list below to load a mapname for <mapnane>

d1_canals_01
d1_canals_02
d1_canals_end
d1_tempanals_02
d1_town_01
d1_town_02
d1_town_03
d1_town_04
d1_town_05
d1_trainstation_01
d1_trainstation_02
d1_trainstation_03
d1_trainstation_05
d1_under_01
d1_under_02
d1_under_03
d1_under_04
d2_coast_01
d2_coast_02
d2_coast_03
d2_coast_04
d2_coast_04_dx60
d2_coast_05
d2_coast_06
d2_coast_07
d2_coast_08
d2_prison_01
d2_prison_02
d2_prison_03
d2_prison_04
d2_prison_05
d3_c17_03
d3_c17_04
d3_c17_05
d3_c17_06a
d3_c17_06b
d3_c17_07

give <itemname> - Substitute <itemname> with a weapon name or item below

weapon_alyxgun
weapon_ar1
weapon_ar2
weapon_bugbait
weapon_cguard
weapon_crowbar
weapon_extinguisher
weapon_flaregun
weapon_frag
weapon_gauss
weapon_hopwire
weapon_iceaxe
weapon_physcannon
weapon_physgun
weapon_pistol
weapon_rpg
weapon_shotgun
weapon_smg1
weapon_smg2
weapon_stickylauncher
weapon_stunstick
weapon_thumper
weapon_sniperrifle
weapon_rollerwand
weapon_molotov
weapon_manhack
weapon_immolator
weapon_irifle
weapon_slam
weapon_hmg1
weapon_cubemap
weapon_binoculars
weapon_ml
weapon_brickbat

Items

item_box_buckshot
item_box_mrounds
item_box_sniper_rounds
item_box_srounds
item_healthkit
item_battery
item_suit
item_ml_grenade
item_ar2_grenade
item_healthvial

More command line Parameters:

Start the game with the -console command line parameter.  Make a shortcut to the game, right click on it and edit the properties to look similar to this:

C:\Program Files\Half-Life 2\hl2.exe" -add command here

-width # -height # Set Width and Height of Window
-fullscreen Full Screen Mode
-heapsize # Set Heap Size
-r_anamorphic 0 Unknown Effect
-netgraph 3 Unknown Effect
-mat_preloadshaders Unknown Effect
-preload Unknown Effect
-dx9 Unknown Effect
-dxlevel 90 Unknown Effect
-mouselook Unknown Effect
-dx8 Unknown Effect
-sw Unknown Effect
-condebug Unknown Effect
-startwindowed Unknown Effect
-window Unknown Effect
-windowed Unknown Effect

Admin console commands (must be your own server or have admin access)

kick - kicks a client off of your server
ban - permanently kick & ban a client off of your server

sv_gravity (works for multiplayer and singleplayer) changes gravity on a server



Links.