Type in these cheat codes while playing: 

RDALL
All items and full health

RDCLIP
Toggle clipping mode

RDDEBUG
Toggle debug mode

RDELVIS
Toggle God mode

RDFUCKNGOXYY
Go to X episode and YY map

RDGUNS
All weapons

RDINVENTORY
All inventory

RDITEMS
All items

RDKEYS
All keys

RDMONSTERS
Toggle no monsters mode

RDRATE
Show frame rate

SHOWMAP
Show map

RDSKILL#
Change skill number # (1 to 4)

RDUNLOCK
Toggle locks