Hiding in plain site? Put on a disguise. Often used to sneak into the evil lair. For best results brain a guard and steal his; no one is tracking these things.
The Faceless Men, A Song of Ice and Fire (GRRM)
Elite assassins who wear the faces of the dead to disguise themselves when they perform their assassinations, and who worship the Many-Faced God.
Peter Pettigrew, Harry Potter and the Prisoner of Azkaban (J. K. Rowling)
Harry Potter has several excellent examples of disguises – the other one which comes to mind is Barty Crouch Jr. spending an entire year as Mad-Eye Moody – but the most masterful disguise award goes to Peter Pettigrew spending 12 years as Scabbers the rat. I remember telling a childhood friend of mine, “You’ll never guess who the villain in the third Harry Potter is!” She insisted that she wanted to know, and so I finally told her it was Scabbers – to which she responded, slightly miffed, “Well, if you didn’t want to tell me you could have just said so.”
The Greek Gods
These guys and gals are always disguising themselves; Athena repeatedly disguises herself as a man during the Odyssey; Demeter as an old woman as she is grieving her daughters loss. Zeus comes to mind most frequently – off the top of my head he turns into a swan, a bull, a golden shower, a satyr, and at least in one operatic rendition (Offenbach’s Orphee aux Enfers), a fly, all with the purpose of variously abducting, raping, or seducing women.
Odysseus, The Penelopiad (Margaret Atwood)
Odysseus returns from his long journey disguised as a homeless man to test the loyalty of his people and servants… and most particularly, to see whether his wife Penelope has been faithful to him in his absence. (Of course, Odysseus himself had no such test – a good thing, because he would have failed it).
Count Olaf, A Series of Unfortunate Events (Lemony Snicket)
A villain known for his many terrible disguises which, somehow, only the Beaudelaires can see through.
Alanna, The Song of the Lioness (Tamora Pierce)
She spends the first two books disguised as a man – “Alan” of Trebond.
Kandra, Mistborn (Brandon Sanderson)
I only remembered them because Lynn included the kandra last week! The kandra are mistwraiths who can take the form of others using their bones – one significant plot in the series involves the revelation that, unbeknownst to our group of heroes, one of them has been killed and replaced by a kandra, and their subsequent attempts to figure out who it is. They are frequently used as spies – for obvious reasons, I’d imagine.