Press ~ and type in Sv_Cheats 1. Then, you can use the following codes:
god: god mode
noclip: disable clipping mode
impulse 101: get all weapons
notarget: enemies don't target you
maps: show map list
budda: invincibility
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
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 #