My most influential MMOG moment was the Shard of the Herald event on Asheron's Call. The devs wanted to move the story forward with the unleashing of an imprisoned demon, but our server decided it would be best to keep that demon chained up! Partly because of lore reasons, but I think mostly because we could!

I think the devs intended there to be a battle for and against it, but I don't think they anticipated an entire month of us successfully protecting that crystal prison.

Before the patch that was scheduled to move the story forward, the developers logged in with several storyline characters and some uber-outfitted and buffed-up rebel players who had earlier pledged allegiance to this demon. The funniest part was a dev who had buffed himself up so much that the stat numbers had actually reset. I would have loved to seen his face when he died as soon as porting into that space. The attackers and devs eventually won, releasing the demon and paving the way for the new patch.

I remember some folks staying up all night in that cavern - random people would attack at odd times trying to throw a wrench in the plans. Other players would bring supplies so that it was manned 24/7. It was just an inspirational event - something totally unexpected and created by the players. The devs did what they had to do, and went about it the right way with using the ingame lore to justify the events rather than just shutting the server down for the patch and pretending everything was set.

That gets my vote.