I remember hearing/seeing on youtube about World of Warcraft having something like that. With all the jumping, you could carefully get higher and higher if you found the right mountain with the right incline. After a while, the guy could jump over the tip of the mountain range, and on the other side was some sort of lava temple from a test thing before it got released. There's a couple of area's like that actually, most reached by exploiting a fast running mount on top of a mountain, jumping and using levitate to glide down over several miles, reaching a part of the mountain that, while not climbable from the bottom, was very climbable from halfway up or so. (what developer would seriously test that beforehand? lol)
I guess they are not technically hidden area's but more like things that they didnt remove after testing, thinking no one would ever be able to reach or see them anyhow.
Only actual thing i can think of at the moment, of a true hidden area with very little purpose, were the Strange Apparatus's from Final Fantasy XI. While i think you all know this, these things were truly hidden, behind a series of fake walls that from the outside look like any other wall, but could be passed through. Once you got through the wall, you encounted a sort of zilart-style hall, with a room at the end of it, holding a strange apparatus. They could be used by having the right title, at the right time, without any real hints to what these were beforehand nor any indication or text proclaiming what needed to be done. Unless you knew about them, or were actually searching for them, there was very little chance you were to ran into one delibirately. And while some might hold -some- use, there were a few that were completely inactive, just there for people to find after exploring. Truly one of my favorite things to find.
[XI] Surivere of Valefor
[XIV] Sir Surian Bedivere of Behemoth