In The Barrens, there is a tombstone complete with a Spirit Healer, known as The Shrine of the Fallen Warrior. This is a shrine dedicated to Michael Koiter, a designer of World of Warcraft who died during development due to heart failure.
The three bankers in Stormwind are named Olivia, Newton and John named after the '80s singer Olivia Newton-John.
In the newest high-level-dungeon "Naxxramas", gamers could sometimes spot a cat running through this dungeon called "Mr. Bigglesworth". A clear pointer to Dr. Evil's cat from the Austin Powers movies.
In Dalaran, at the top of the Violet Citadel tower, there is a NPC nammed Alfred Copperworth, a reference to Alfred Pennyworth from the Batman comics.
In the entrance to the Mana Tombs, there is a shady dealer by the name of "Slim". This is a reference to Eminem aka Slim Shady.