MegaMod is a famous collection of some of the better and well-tested mods for Master of Orion 3. It includes purely cosmetic changes such as nicer UI graphics, cleaned up text, and many many changes to game and race balance. Some existing bugs are worked around; for instance, racial Ground Combat picks are now hardcoded since they originally did not work at all. Poke around; you'll find many pleasant changes.
After this version, MegaMod began to require a specially modified version of the MoO3 1.2.5 executable. Since no one has been bold enough to try to duplicate these modifications on MacOS, version 5.5 is probably the last one we can use.
I have taken the liberty of including an updated version of the Encyclopedia Mod (4.3) and the BaseRelations.txt fix.
Some information on MegaMod 5.5's contents can be found here:
Classic and Mac OS X
Working install of Master of Orion 3