Bloody tip
Go to the 1st level of the 1st episode. Stand at the road where the truck rides in circles. Push an alien onto the road, you can push him if he doesn't shoot. The result will be a nice splash of blood. The alien will fly up and will splash in parts when it hits the ground.
Codes
RDELVIS Invincibility toggle
RDHOUNDDOG Invincibility toggle
RDALL Gives you all health, items, ammo and weapons
RDGUNS All weapons
RDINVENTORY All inventory
RDITEMS All items
RDKEYS All keys
SHOWMAP Full map
RDCLIP No clipping mode
RDMONSTERS Enemy toggle
RDSKILL# Restart level at skill # (1-4)
RDRATE Display frame rate
RDVIEW Chase plane view
RDUNLOCK Toggle locks
RDDEBUG Debug mode toggle
RDBETA Displays "Eatme!"
RDMAXX Displays "MAXX Rules"
RDRAFAEL Displays "For you Granpa!"
RDMOONSHINE ???