Cross-Realm Orgrimmar / Stormwind

This is quite simple.

For this scenario, let's assume you are on Realm A, and your Battletag/RealID friend is on Realm B.

• Step 1) Invite friend from Realm B to a group
• Step 2) Both stand outside SW/Org gates
• Step 3) Pet-Battle duel them - ask them not to accept it
• Step 4) They can walk right into your Stormwind and they'll be in Realm A's Stormwind. All players are visibe. You can attack them, etc. 

Good for small WPvP, RP, possibly other things. Who knows.

May be able to extend it into something better.