During gameplay press ~ key to open the console then type :

give x

where x is

Code - Effect

weapon_thunder - Electro Gun
ammo_thunder - Electrogun and flame thrower ammo
weapon_thunder_fire - Flame thrower
weapon_grenade - Grenade
weapon_maxim - Heavy Machine Gun
ammo_maxim - Heavy Machine Gun ammo
item_medikit_large - Large medikit
weapon_bottle - Molotov Cocktail
weapon_nailgun - Nailgun
ammo_nailgun - Nailgun ammo
weapon_mauzer - Pistol
ammo_mauzer - Pistol ammo
weapon_rifle - Rifle
ammo_rifle - Rifle & sniper rifle ammo
weapon_obrez - Shotgun
ammo_obrez - Shotgun ammo
item_medikit_small - Small medikit
weapon_rifle_sniper - Sniper rifle
weapon_machinegun - Sub Machine Gun
ammo_ppsh - Sub Machine Gun ammo
weapon_spanner - Wrench