Open console with the ~ key then type one of the following

REB GOD - God Mode
REB INFINITE AMMO - Unlimited Ammo
NEXT LEVEL - Skip current level
GIVE [ITEM] - Get Item
GIVE AND EQUIP [ITEM] - Give & Equip (See list below)

Items:

LASER
BINARY_LASER
MAXIM_LASER
BATTERY_PLUS
PLASMA_PISTOL
PLASMATRIX
PLASMAGNUM
AUTOLOCK_BOLTS
GRENADE_LAUNCHER
GRENADE_PLUS
GRENADE_EMP
MISSILE LAUNCHER
MISSILE_PLUS
MISSILE_EMP
FLAMETHROWER
NANOFRAG
EPULSAR

Keys:

KEY
KEYB

Shield:

SHIELD_PICKUP = Standard Shield
ARMOUOR_PICKUP = Heavy Shield

Body Slot Upgrade Pickup:

AIM_ACCURACY_PICKUP
LOCK_DECODER
BEACON_TRACKER
TERRAIN_SCANNER
TERRAIN_SCANNER_WITH_RADAR
OMNI_SCANNER
HOLOGRAM_GENERATOR
AUDIO_CLOAK
SIGHT_PICKUP
SIGHTPLUS_PICKUP