Press ~ during game play to display the console window, then type
sv_cheats  1  to  enable  cheat mode. Then, enter one of the following
codes at the console window to activate the cheat function.

Cheat code:             Effect:
sv_gravity 2            High jumps
buddha                  Invincibility
impulse 102             Brain target practice
impulse 101             More weapons
noclip                  No clipping mode

Other cheats:
Press ~ and type this:

give weapon_stunstick
give weapon_katana