So, you're stumped on a particular map and cannot advance any further?  Well, you can find passwords for all the maps further down on this page. But first, here are some codes that will work with both the Japanese and North American versions of this game...please note that these codes work with individual maps only--they cannot be used during a campaign (thank goodness, you cheating bastard).

Play as the XENON (GUICY)  instead of the UNION  (UNION) :
On the map selection screen, press and hold   L1  +  L2  +  CIRCLE  and then press X.  You cannot play an entire campaign as the XENON army, but you can play individual maps.

Watch the computer fight itself :
I have yet to confirm that this can be done in the PlayStation version.  I suspect it can be done, but it wouldn't be the first time that a "classic" code was left out of a game in the series:  the Gameboy release (Nectaris GB) doesn't allow ANY cheats--or, more precisely, I have yet to discover them, since the usual sequence of buttons yields nothing.

Two-player VERSUS mode :
On the map selection screen, press and hold  L1  +  L2  +  SQUARE  and then press X.  Normally, this feature is UNLOCKED by completing both the "New Story" and "Legend Story" campaigns in their entirety (64 maps).
 
DOUBLE your troops' attack power :
On the map selection screen, press and hold  R1  and then press X.   Note: in order to use this code, you must have already successfully completed both of the main campaigns.
 
Single scenario mode :
Well, this isn't a cheat.  This mode is UNLOCKED upon successfully completing both main campaigns.  The two-player versus mode is similarly unlocked.
 
BATTLE TEST mode :
Again, this mode is only available once you successfully complete both of the main campaigns.  Enter "BATTLE" as a password and you will be able to pit various troops against one another in battle simulations.  Can a squad of infantry hold their own against a pack of HUNTERS?

Here are the passwords to all 64 maps in Nectaris (PSX) that require passwords (note: the 44 "Contest Winner" maps do not require passwords). The 32 maps comprising "New Story" are exclusive to PSX Nectaris--this campaign is not  available anywhere else--unlike the 44 "Contest Winner" maps which are  also available on the Gameboy title (Nectaris GB); or the ubiquitous "Legend Story" campaign which is available on each and every Nectaris title ever released (whether you should consider this lazy recycling of content or a nice bonus that respects the game's roots is for you to decide).  Personally, I think Hudson Soft should have thrown-in additional "unlockable" maps (either brand new maps or recycled ones from Neo Nectaris) for the hardcore fans who have completed the "Legend Story" campaign one too many times.  I would have loved to unlock some "Honorable Mention" maps--you know, the contest entries that didn't make the final cut... that would have been a nice and easy way for Hudson to provide more content.  Better still, I would have liked to see some "Contest Loser" maps that were rejected because they were downright awful / flawed / etc.  Oh well, maybe next time...

TRIVIA: If you are a fan of the TG-16, you may have noticed that the classic campaign on this PlayStation title has unfamiliar map names.  The discrepancy is easily explained:  the PlayStation's "Legend Story" is taken straight from the original PC Engine game.  When Nectaris was released in North America for the TurboGrafx-16 under the title "Military Madness", 18 passwords were altered. For example, Map 9 changed from "GALOIS" to "GALIOS" and map 14 "APPOLO" was corrected to "APOLLO". In addition, the passwords for the entire advanced campaign (maps 17-32) were changed for the TG-16 title, as follows:

17. MILTON   18. IRAGAN   19. LIPTUS   20. INAKKA   21. TETROS   22. ARBINE   23. RECTOS   24. YEANTA
25. MONOGA   26. ATTAYA   27. DESHTA   28. NEKOSE   29. ERATIN   30. SOLCIS   31. SAGINE   32. WINNER

Needless to say, the passwords listed above only work with the TG-16 game; when Jaleco released Nectaris: Military Madness for PlayStation in North America, it left the Japanese map names intact.  Some online resources have confounded the TG-16 & Japanese map names from the two games--don't be confused any longer!