I'm a big fan of 'treasure hunting', and so I like to try and find super rare items that are very rarely seen, that of course often carry a high value as a result. I'd like this post to be a resource of sorts for people to refer back to, to have lots of information all in one place, so I'll start the ball rolling by adding an entry:
Source: random mob drop in Uldaman (probably just Troggs, but so rare as to not be verified at present)
Value: approximately 100,000 gold, though again this can vary wildly because of rarity
This is an incredibly rare drop from Uldaman. I've been lucky enough to get two of these in the past from my intensive farming sessions, but it is by far one of the hardest items I've actually managed to get. It's up there with glorious legplates, if not even harder to farm because of the lack of mobs that can actually drop this.
I look forward to reading about other items! Thanks in advance.
EDIT: Thanks for the great feedback so far. I hope this becomes a valuable resource for you goblins who, like me, love to go out and get the rarest treasure possible!