Don't know how known this is but I thought I should post something me and a friend have been doing.
You need a Druid and a Rogue, on separate accounts.
If they are on separate factions you can explore inside cities, if they are on the same faction you can only explore where you can duel.
Basically, what you do is make the Rogue spec into Shadowstep, and the Druid spec into Force of Nature.
Start a duel or turn on PvP, summon Treants on top of a roof or cliff or somewhere you want to access, and Shadowstep to the Treants.
The Druid can (usually) Feral Charge(Cat) the Rogue and climb even higher, but Ive found that one well-placed Treant-step is enough for most buildings/hills.
If this is very well known then I guess we can use this thread to discuss areas where this is useful. Ill post a few areas that I have found.
Stormwind Cathedral (you can get to the top of the tower, but this looked cooler for a piccy!)
Any rooftop in Stormwind (can probably get to the gated areas easily from here)
On top of the cliffs surrounding Orgrimmar. (this one is overlooking the Zepelin to STV/UC, and you can jump on top of them using Slowfall)
On top of the Zepelin in question.
There aren't many limitations, but exploring this way can be kind of slow due to the long cooldown on Force of Nature. There are alot of places that you can get to in only one jump, but I always find it hard to think of good ones whenever I stumble upon a new method.
Can I have any suggestions on where this method can get you?