There is the official game: Populous: The Beginning (populous 3)
There is 1 official expansion: Undiscovered Worlds
There are currently 15 unofficial expansions (mods) avaliable
------------------------------------------------------------------------------------------------------------------------------------
------------------------------------------------------------------------------------------------------------------------------------
Populous: The Beginning (OFFICIAL)
Populous: The Beginning Undiscovered Worlds (OFFICIAL)
Populous: Age of Chaos (UNOFFICIAL)
Populous: Katara's Voyage (UNOFFICIAL)
Populous: Tikal's Journey (UNOFFICIAL)
Populous: The Solar System Of Devil Worlds (UNOFFICIAL)
Populous: Original Worlds (UNOFFICIAL)
Populous: Multiverse (UNOFFICIAL)
Populous: Anniversary (UNOFFICIAL)
Populous: JC Levels (UNOFFICIAL)
Populous: The Order System (UNOFFICIAL)
Populous: War for Survival (UNOFFICIAL)
Populous: Path of the Godchosen (UNOFFICIAL)
Populous: Countdown Challenge (UNOFFICIAL)
Populous: Mini Games (UNOFFICIAL)
Populous: Another Journey (UNOFFICIAL)
Populous: Forge of Fates (UNOFFICIAL)
Populous: Odd Worlds (UNOFFICIAL)
Populous: Race Challenge (UNOFFICIAL)
Populous: The Summoning I (UNOFFICIAL)
Populous: The Summoning II (UNOFFICIAL)
Populous: The Rescue (UNOFFICIAL)
...and more mods being made...
------------------------------------------------------------------------------------------------------------------------------------
------------------------------------------------------------------------------------------------------------------------------------
PopRe Site to download the game, its expansions, forums, to play online, and much more!
All campaigns also free to DL here Pop Campaigns.