In the frozen wastes surrounding the Akali Lake Weapon X facility, Wolverine stumbles upon an artifact of untold power from another world...of Warcraft.
I'm placing the main image below in case the spoiler warning tag isn't enough to keep you from peeking inside. It really is one of the more humorous little cross-title nods I've come across lately, and I certainly wouldn't want to spoil the surprise for anyone else. Let's just say that millions of people across the world will recognize what they are seeing immediately upon finding it and leave it at that. Now go away now if you don't want to know. Seriously.
Given you plenty of time to leave now. Without further adieu, here's a shot of the hidden area I stumbled upon while playing X-Men Origins: Wolverine.
Not only did this discovery nearly make me piss myself laughing, it also scored me an achievement, aptly titled, "WoW!". with the description, "You feel cold as you examine the skeleton and read the name "Arthas" etched into the nearby sword."
Technically not a spoiler really, seeing as it's been outed on achievement lists all over the internet, but nonetheless an amusing little nod to one of Activision Blizzard's slightly more popular franchises.
UPDATE: Here's a pic of another hidden location that a reader sent in, which unlocks the achievement "Found!". I wonder what's under that hatch?
And here's a hi-rez screen of the Frostmourne scene, complete with Wolverine in his unlockable Legendary costume.