The first online RPG I ever really got into, and which gave me a taste for the genre that hasn't left me yet, I really loved this Euro sci-fi-with-an-eco-bent game, which sadly never really took off in the way other MMORPGs have, like 'WoW', 'Everquest' and the much-missed 'Star Wars Galaxies'.
It was 'Ryzom', by a little-known Euro game developer called Nevrax.
It was pretty unique at the time, as it featured real-life changing seasons which affected both the resources you could gather and the animal behaviour, which I don’t think has ever been replicated before (World of Warcraft has ‘seasonal events’ for Christmas, etc, but outside of that, the zones and resources do not change).
The animals also interacted with one another, predators taking down prey animals, etc. Something I've always wished would be implemented in next month's choice. It was a great match for my newly purchased Dell beast, optimised for gaming, which handled the beautiful graphics with aplomb.
Wiki tells me its still available. Maybe if I have more free time one day, I'll go back...