If there is one thing to say about the World of Warcraft, it is that it is enormous and seamless. Blizzard loves Easter eggs and has hidden secrets and secret areas all across the entire world. It goes without saying that you have found at least one or two of these secret areas. Some people even make it a hobby of finding and claiming secret areas.
Note that there is a difference between secret areas and areas only reachable by exploit. The first is intended by Blizzard, but the second is a bannable offense. Do not try and exploit your way to Old Ironforge (especially since you can just walk in now as of 4.1!), and do not try to enter Mount Hyjal, or else the Guardians of Bliz-- okay, so maybe my knowledge of secret areas is a bit dated, but I still enjoy the hunt for them!
Some guilds, especially on RP realms, even use and claim secret areas and locations as their headquarters and sometimes even hold guild meetings in these lcoations. This is a great alternative to player housing, and Blizzard actually encourages people and guilds to find and enjoy such fantastic locations. With Deathwing's emergence and flying mounts now available in Kalimdor and the Eastern Kingdoms, previously unreachable areas are now just a short flight away, so there are more tucked-away locations than ever!
What secret locations have you found, and which is your favorite? Do you often go there? Does your guild have any special spot to claim as its own?