While playing type any of these codes and have a blast.

Code Effect

IDDQD Invulnerability
IDBEHOLDx Gain temporary ability. x can be:
I=invisibility R=radition suit
S=berserk A=automap
V=invulnerability L=lightamps
IDKFA Full health
ammo
weapons
armor and keys
IDFA As above
but no keys
IDDT While viewing map. 1x= all map 2x=more details
IDCHOPPERS Gain chainsaw
IDCLEVxx Warp to level 'xx'
IDMYPOS Displays your position and bearing
IDMUSxx Music select. 'xx' is the level who's song you want.
IDSPISPOPD Walk through walls!