All Ammunition
Select the Enter Code option from the main menu, then enter SLGFST.
(Thanks to Kallenw91@aol.com)

All Weapons
Select the Enter Code option from the main menu, then enter GBZRK.
(Thanks to Kallenw91@aol.com)

Level Passwords
Level 2 (Bridge): FLLNGDWN
Level 3 (Fridge): GTMLK
Level 4 (Freezer): CHLLBB
Level 5 (Inside Wall): CLSNGN
Level 6 (Graveyard): DGTHS
Level 7 (Castle): RNKNSTN
Level 8 (Tan Base): BDBZ
Level 9 (Revenge): LBBCK
Level 10 (Desk): DSKJB
Level 11 (Bed): GTSLP
Level 12 (Blue Town): SMLLVLL
Level 13 (Cashier): CHRGT
Level 14 (Train): NTBRT
Level 15 (Rockets): RDGLR
Level 16 (Pool Table): FSTNLS
Level 17 (Pinball): WHSWZRD
(Thanks to armymen17486@aol.com)