For those who already played through all the adventures in Zeus and Poseidon, there is more:

Impressions released a new adventures called "Rich Man's Blues" for Poseidon. It's not included in the GoG-Release, but is free for all customers. You can get it here: http://www.sierrahelp.com/Patches-Updates/Patches-Updates-Games/ZeusUpdates.html
~600kb/2mb in disc
If you own the german retail version, you can also get a german version of the adventure: http://www.villages-and-cities.de/downloads/patches/DasVersprechendesHades.EXE
After installation, it is locaded under "custom adventures"

But with the editor, there are a lot of fan-made adventures.
Over 100 for Zeus: http://zeus.heavengames.com/downloads/lister.php?category=adventures
Over 300 for Poseidon: http://zeus.heavengames.com/downloads/lister.php?category=adventures_pos
More, but only in german: http://www.villages-and-cities.de/downloads/missions/gadt/download.html

The german adventures should run with the GoG Version, besides they obvious difference in language. I haven't played a lot of the fan-made adventures and this was a long time ago, so I can't really recommend any. Feel free to recommend some in this thread.

Very cool, thank you!

Zeus Poseidon game, I used to have the actual game on discs years ago & decided to play it once more, now I have created custom adventures but do not find them when I go into game play! Can you help me?