Enter the following at the Secret Unlocks screen:

PASSPORT    Level select
MELTDOWN    Chain Reaction level
BLASTOFF    Countdown level
AQUA        Deep Descent level
VACUUM      Equinox level
FLAME       Phoenix Fire level
MAGAZINE    Bigger clip for sniper rifle
SLEEPY      Upgraded tranquilizer darts
TARGET      Platinum medal
Q LAB       All gadget upgrades
ZAP         Stunner upgrade
LIFTOFF     Grapple upgrade
PHOTON      Laser upgrade
AU PP7      Golden Guns
SCOPE       Multiplayer Assassination option 
TNT         Multiplayer Demolition option
BOOM        Multiplayer Explosive Scenery option 
ORBIT       Multiplayer GoldenEye Strike option 
GUARDIAN    Multiplayer Protect option
TEAMWORK    Multiplayer Team King Of The Hill option
TRANSMIT    Multiplayer Uplink option
PARTY       Multiplayer All Characters
VOODOO      Multiplayer Baron Samedi
BLACKTIE    Multiplayer Bond Tuxedo
NUCLEAR     Multiplayer mode Christmas Jones
MIDAS       Multiplayer mode Goldfinger
DENTAL      Multiplayer mode Jaws
BLIMP       Multiplayer mode Max Zorin
PHOTON      Multiplayer mode May Day
BOWLER      Multiplayer mode Oddjob
CIRCUS      Multiplayer mode Pussy Galore
ASSASSIN    Multiplayer mode Scaramanga
JANUS       Multiplayer mode Xenia Onatopp
GAMEROOM    Get all Multiplayer Options

Get a Shelby Cobra:

Play the ‘Enemies Vanquished’ level. Pause the game, hold L and press Right,

Right, Left, Left, Up and release L.

Thanks to Revolution readers Mario, Dave Toenail, and Marty Martin!