From
Apple Museum
System 6
MultiFinder: Juggler, Oggler, Twitcher
System 6.04: Antares
System 6.05: Big Deal
System 6.06: SixPack
System 6.08: Terminator (it terminated the System 6 era)
System 7
Finder 7.0: Furnishings 2000 (a defunct San Francisco Bay Area furniture store)
System 7: Blue (System 7 programmers were called "blue meanies"), Big Bang, M80 (a powerful firecracker), Pleiades
System 7 Tune-Up: 7-Up
System 7.01: Road Warrior (used in first PowerBooks), Beta Cheese
System 7.1: Cube-E, I Tripoli (both because the project was to conform to IEEE standards)
System 7.1 Pro: Jirocho
System 7.5: Mozart, Capone (Apple hoped that like the gangster, "Capone" would rule over "Chicago", the code name for Windows 95)
System 7.5 Update 1.0: Danook (from Gary Larson's Far Side cartoon)
System 7.5 Update 2.0: Thag (also from Gary Larson's Far Side cartoon)
System 7.52: Marconi (named after Guglielmo Marconi, 19th century Italian engineer)
System 7.53: Unity (it united all the patches and special software of all models into one update)
System 7.53 Revision 1: Buster (CEO Gil Amelio's high school nickname, used often in later Mac OS versions)
System 7.53 Revision 2: Son of Buster
Mac OS 7.6: Harmony
Mac OS 7.61: Ides of Buster
Mac OS 8
Mac OS 8: Tempo
Mac OS 8.1: Bride of Buster
Mac OS 8.5: Allegro, Scimitar (beta version)
Mac OS 8.6: Veronica
Mac OS in '99: Sonata
Mac OS X
Mac OS X Server: Rhapsody
Mac OS X Server DR2: Titan
Mac OS X Server CR1: Mac OS Enterprise ("Enterprise" is also the name of Apple's NeXT division)
Mac OS X: Cyan (a different "shade" of the Blue Box), Siam (like Siamese twins, Mac OS and Rhapsody will be joined)
Mac OS X API's: Carbon (all Mac life will be based on it)
Other OS Projects
Mac OS for Intel: Star Trek (probably because the Mac OS is boldly going where it has never gone before)
defunct Apple/IBM object-oriented OS: Defiant, Pink
defunct System 8: Copland (named after Aaron Copland, musician), Maxwell (internal code name)
defunct System 9: Gershwin (named after George Gershwin, musician)
Copland Interface Project: MUSE (for Maxwell USer Experience)
CHRP compliant Mac OS 8: Orient Express