This post is dedicated to getting to secret areas of the game that are normally unreachable, or are just way out of the way and not known by too many people. If you know of any that aren't explained here, tell us. Throughout this page I use the term "fancy jump". What this means is that you position yourself against the wall at the proper angle, jump in the proper direction, and hit the wall at the right height so that you are able to latch onto an invisible ledge. This takes practice. "Fancy jumping" is easiest to do with Night Elves, as their body shape latches better than others. Hardest to do this with is gnomes. Each and every one of these has been personally tested by me before being posted. Now, let's get to some exploits.
GM Island has always been one of the most talked about exploits. Supposedly you can only get there by hacking the system or using a private server. I touch more on the subject of getting there further down this page.
Hyjal. A secret zone just west of Winterspring. Location of the World Tree and the Well of Eternity. Completely unreachable by normal means. At approximately 52,84, there's a cave. If you go into it you will see a walled off instance. That's how you know this is Hyjal. Go out, and go left (if you are facing the cave). From here you can climb up one of the nearby pillars, onto the mountains, and eventually get on .
If you want to see GM Island, I would recommend checking out one of those "offline" WoW servers (google is your friend, I can't help you find one), as you will be able to use a teleportation command to send you to the island. You can also use model/map viewing tools to view the island, as all the map for it is stored on your client. Of course, use of such servers and tools may be a violation of WoW's EULA and the Terms of Service, but seeing as you really want to just see the island I doubt you care :)
You could always apply for a job at Bliz, that would be another way to see it.