Press ~ (tilde) to bring up the 
console window and use the codes below. 

killpawns - Kill All Enemies 
setjumpz # - Set Jump Height (500 = Normal) 
setspeed # - Set Speed (2 = Normal) 
xgod - God Mode 
xfly - Fly Mode 
xghost - No Clipping 
xwalk - No Clipping/Fly Mode Off