Enter W93M 7H2O BCYO PSVB. This code will give you 100 health, 200 armour, all weapons, full ammo and a backpack. It'll also supply you with the three pentagram icons that make your laser three times stronger than the BFG and can kill bosses in four seconds flat. These useful pentagrams also let you use the three mysterious switches in the last level, closing the gates which spawn the monsters.
The Terraformer - CB92 NBPL SYL? JO27
Main Engineering - BXYH ?G41 6Z4J PJ?Z
Holding Area - CYCC MGPK X47G TS2B
Tech Center - CF3? PG6D S12Z PFKB
Alpha Quadrant - BXRO TH1F 52GG 7W?B
Research Lab - BBXW HLGS XB8F 4RKB
Final Outpost - FVV9 FL55 QGFV DWJB
Even Simpler - FFLB MQ6C VV1C PF1B
Very cool DOOM cheat
In Doom 64, enter this password ?TJL-BDFW-BFGV-JVVB. At any time during a level, pause it and there will be a new option called FEATURES. IT will give things like invincibility, all weapons, etc. In the first feature choose the level you want and press a C button to go to it.