Open up your hitman.ini file located in your Hitman directory. Add the line 'EnableCheats 1' to the file. When playing a mission, hit the '~' key. Types any of these codes:
(SPECIAL NOTE: Some have noted that you can simply type the codes in while playing (with CAPS LOCK on) instead of using the console. You can also try adding the line 'EnableConsole 1' to the hitman.ini file to enable the use of the console.)
Code - Result:
IOIRULEZ - God Mode
IOIGIVES - All Weapons with Max Ammo
IOIHITLEIF - Full Health
IOIER - Bomb Mode
IOISLO - Slo-Mo Mode
IOIHITALI - Ali Mode
IOILEPOW - Lethal Charge Mode
IOIGRV - Gravitation
IOINGUN - Nailgun Mode
IOIPOWER - Mega-Force

Access All Missions
Open up your hitman2.ini file and change:
DefaultScene=AllLevels/logos.gms
to
DefaultScene=AllLevels/levelmenu.gms
Start the game and you will get a level select menu, with all missions unlocked.