Doom 64

Enter: W93M 7H2O BCYO PSVB
This code gives you 100 health, 200 armor,
all weapons, full ammo, the backpack, levels
29-31 finished, and takes you to the last level.
Additionally, it gives you three pentagram items
that make your laser three times as powerful as
the BFG9000. The three pentagram items also
enable you to use the three switches in the last
level (and closes the gates that spawn the monsters).

The big one!
This code enables all the big ones: God Mode, all levels,
all weapons, all ammo, full maps, and full health.
Enter: ?TJL BDFW BFGV JVVB
Go to Features in the main menu and you'll have
your choices in front of you.

Taunts
After you've died in Doom 64, text-style taunts
will appear on your screen direct from the Midway
developers. Wait about 25 seconds after dying to
see them. Some of these "taunts" are very amusing.