Press ~ to go to the console, then type:

Give all = give's you all avaliable weapon's and extra's (999 ammo)
No target = no bad guys can see you
Noclip = walk through wall's (cool 3D lay out)
Give rocketlauncher: Rocket Launcher
Give rockets x: x is number of rockets you want
Give grenadelauncher: Grenade Launcher
Give grenades x: x is number of grenades you want
Give shotgun: Shotgun
Give shells x: x is the number of shells you want
Give machinegun: Gives Machine Gun
Give bullets x: x is number of bullets you want
God: God Mode