The Big O. Subverted in that while she's called Angel and even has two scars on her back where there were once wings, she's not exactly an angel.
El Cazador de la Bruja. In the middle of Mexico/South of Mexico, there is an inn run by an old man; he stops the villains chasing the heroines for a day or two so that they can have some character development. This includes a Tyke Bomb and witch with magic powers, who he stops simply by looking at them. Apparently is really the Hopi Fertility Deity Kokopelli. He takes the form of a white author who died 3 years prior to the plot.
In a somewhat... different example of this trope, Rei Ayanami of Neon Genesis Evangelion is implied - but never outright stated - to be a somewhat different breed of "Angel". And while you are chewing on that and that infamous ending and movie sequel, somehow Anno manages to sneak in a little piece of information that everybody is an angel, since humanity descended from the second angel, Lilith, just like all the other angels descend from Adam.
In Love Actually, Rowan Atkinson's character is an angel, though it was more explicit in the original script. The only thing that was altered in his 'storyline' is a shot of him fading away as he walks off at the end.
In Van Helsing the title character is implied to be one of these. He is in the service of God, is apparently immortal (he remembers fighting Romans at Masada, and was Dracula's murderer hundreds of years prior to the movie), and in the novelization he is said to have two scars on his back where wings may have been. Furthermore, Dracula refers to him as "Gabriel."
The hospitaller in Kingdom Of Heaven is implied to be this in the extended cut. According to the DVD Commentary, he may even be God himself. There are a number of hints throughout the movie, such a bush catching on fire just as he appears, and telling Balian that if God has purpose for him, he will keep him safe—right after which Balian is the only one to survive a shipwreck. It's never explicitly stated, and the filmmakers even kept it secret from the actor who portrayed him, although they admit he probably figured it out anyway. The character is also a mouthpiece for a more modern and tolerant depiction of Christianity than the bloodthirsty bigots that make up most of the cast, including much that would have been hugely heretical at the time.
In Almost An Angel, former bank robber Terry Dean is convinced that he's died and been sent back to earth as an angel. Most of the other characters think he's just a eccentric, but accept him since he's a pretty nice guy despite the crazy. The last few minutes of the movie reveal that, indeed, he's an angel for real.
Mary Poppins: She's seen putting her makeup on while sitting waist-deep in a cloudbank, for heaven's sake.
The titular character in The Legend Of Bagger Vance is subtly hinted to be one of these. The most overt example is when he says there's a storm coming on a perfectly clear day, which later develops a storm that prevents the hero from running out of town.
Pale Rider: The "Preacher" rides out of the mountains on a pale horse as Megan prays for help against Lahood's men, and there are heavy implications that he is a dead gunfighter sent back to Earth.
John Coffey in The Green Mile may or may not be an angel (if he isn't Christ).
Constantine provides some backstory about the rules of the Cold War between Heaven and Hell. Neither side is able to set foot in the real world, but "half breeds" on either side are able to. They don't do much directly, instead influencing mortals to act on their behalf. With a whispered word they could give someone endless Heroic Resolve, or drive them straight over the Despair Event Horizon. The Stinger implies that Chaz was one all along.
Lani, from What Happened to Lani Garver is a Magical Queer come to help the protagonist sort out her life, and who may or may not actually be an angel.
The wizards in The Lord of the Rings fit this trope. They appear to be Men, but have magical powers and are very, very old and mysterious. Pippin at one point wonders just what Gandalf is, but gets distracted. Digging through the appendices and the Silmarillion, the dedicated reader can find that the five "Wizards" (the Istari) are a group of Maiar sent to help the peoples of Middle-earth - essentially angels of the same order as Sauron. Two are never named: the other three are Gandalf, Radagast, and Saruman.
And of course, Tolkien directly compared them to angels in his letters.
The unnamed ones are named in Unfinished Tales: Alatar and Pallando (or Morinehtar and Romest, as in the History of Middle-earth?), the Blue Wizards that disappeared into the east of Middle-earth to combat Sauron by encouraging uprising and rebellion against him.
And that literal angel's own words imply that Mouse is also this trope, albeit in dogasaurus-form rather than human and an Asian version, namely a foo dog.
Apparently, Halloween was an attempt to prevent mortals dying during a particularly powerful time of the year magically, when immortals come to feed. But, if everyone is wearing a mask, there's a strong possibility that you're about to try and eat someone your own psychic size. Hilarious!
Michael Valentine Smith from Stranger In A Strange Land by Robert Heinlein is strongly implied to be The Archangel Michael of the Bible. (It's never quite stated outright, but the particular way in which we never see both at the same time — with the angel Michael being mysteriously absent from what we see of Heaven for most of the book — strongly suggests the connection. Or else a suspiciously plot-convenient coincidence, of course.)
In Heilein's Magic Inc it gradually becomes clearer and clearer that the witch in the story is no mere with but the Earth Mother/Gaia herself.
Reorx often walks Krynn disguised as a rather fashionable dwarf named Dugan Redhammer, often revealing himself at the end of the story.
There is a short story by Isaac Asimov, where a character is revealed to be the Devil in the end - and it is unclear whether he is aware of that.
Implied with Scylla from The Darksword Trilogy. She says she's a secret agent, but always flashes her ID card too fast for anyone to see exactly which agency she's an agent of. Later, it is said that the only ones who can use Time magic, as Scylla has done in the story, are Agents of God. (Or the Almin, as the series' God is known).
All through Dan Simmons' Hyperion Cantos, A Bettik, the last surviving android in the galaxy is ordered about by every human in the series. It's only at the end of the final book that he is revealed to be an observer created by the entities who had been driving events from their home in hyperspace.
Occurs in one of the stories in the children's book The Ten Tales Of Shellover - an old man finds a starving cat in the snow and takes her in. She eats all his bread, milk and meat, and makes him use up all his logs, but he doesn't throw her out. In the end, she leaves, and his milk, bread, meat and logs never run out from that day forth. Interestingly, he may suspect that she isn't an ordinary cat when she asks him why he doesn't drive her away and leaves no footprints.
Central theme in Leo Tolstoy's short story "Where Love is, There God is Also." Martin, a poor cobbler who lost his son years ago has a vision one day that he would be visited by Jesus Christ in course of the day. In course of that day, he is visited by a number of people in unfortunate circumstances whom he treats kindly. At the end of the day, he realizes that he has been visited by Jesus, after all.
It's implied that the hallucinatory guides of multiple characters are something like this. They are visible to whoever they want and corporeal whenever they want. In the end, as Virtual Six and Virtual Baltar talk, Six says they work for God and Baltar says the entity they work for does not care for that name. Which basically means it's either the/a god with a sense of humor, or a sufficiently advanced alien being/machine/whatever that some of these "virtual beings" deify.
Anna spent some time in mental institutions as a schizophrenic patient. She fell to become human, was born as a baby and grew up, then regained her own grace.
Archangel Gabriel, who had been hiding as The Trickster since seasons prior. He has been hiding on Earth for millenia and done such a good job of it that other magical beings and even non-Christian gods do not realize that he is an angel and not one of them.
The season 5 finale: Chuck finishes writing his story and vanishes with a knowing smile on his face. Debate is raging in the fandom about whether this means the writer was literally God.
In the finale of Ashes to Ashes, Gene Hunt is revealed as a deceased human who, although having no conscious knowledge of it, has acted as a guide and protector of the other cops stumbling across him in cop purgatory. He fully and knowingly takes up the mantle by series' end.
Bones had an episode where Brennan talks, late at night while alone in the lab, with a janitor named Micah. Considering that this character never appeared on the show before or after, no one seemed to see him except Brennan, and the name does refer to a Biblical prophet, there was some speculation about him.
The Eighties Matchbox B-Line Disaster's song "I Could be an Angle" is based on the misspelled sign of a beggar who was trying to invoke this trope.
This idea is the main theme of Michael W. Smith's "Angels Unaware," undoubtedly inspired by the Trope Namer:
Maybe there is more than meets the eye Who's that stranger there beside you? Don't be smug and don't be cruel Maybe we are entertaining angels unaware.
Mythology and Religion
In Norse Mythology, Odin was known to wander around as an old traveler. He tended to wear a blue cloak and a hat that was pulled down to hide his missing eye. This avatar, Grimnir, is often considered to be the origin of the classic image of the wizard. Some examples from Old Norse literature:
In Völsunga saga, Odin in disguise visits Sigurd repeatedly to give him valuable advice.
In Hrolf Kraki's Saga, a magically skilled, one-eyed Swedish farmer called Hrani gives advice to Hrolf Kraki and his champions that helps them overcome their opponent King Adils. On their way home, Hrani wants to present them weapons, yet they decline. Only afterwards it dawns on them that Hrani was Odin.
You must appreciate exactly how often this sort of thing happens in Greek and Roman tradition. Even the fact that Evander treated Heracles nicely as a stranger was treated as a throw away line in the Aeneid.
The Bible quote above refers to the hospitality that Abraham offered angels, making this Older Than Feudalism. (Genesis 18:2-16). Unfortunately this is kind of a Broken Aesop - you're told to make your house open to all, but only because of the possible promise of a great reward later.
According to the Book of Acts, Paul and Silas were taken to be Zeus and Hermes in disguise during their travels. Denying it got them in a LOT of trouble.
In the story of Lot, two angels visit the city of Sodom and are put up for the night by Lot and his family, who protects his divine guests when his neighbors want to rape them in a mob. Lot refuses to let the mob do so, even offering his own daughters instead, but the mob refuses. (Genesis 19:1-11) Lot offering his daughters in exchange isn't seen all that favourably by many people.
See also the deuterocanonical Book of Tobit, which involved him traveling around with Raphael, one of the archangels. When he first appears, the Archangel Raphael is disguised as Azarias, the son of the great Ananias, and is seen traveling with Tobias. After traveling a bit, Raphael proceeds to show him how to drive off the demon Asmodeus, who had killed the seven men Tobias' bride Sarah had married before, after which he(Raphael) bound the demon. He also showed Tobias’ how to cure his father’s(Tobit’s) blindness, before revealing himself as the Archangel. It's a cool story.
Jacob/Israel had a wrestling match with an angel.
And in the New Testament we have the two men discussing Jesus' death and rumors of his resurrection while walking to Emmaus. A third man joins them and explains the whole deal...guess who he was?
Jewish tradition has the prophet Elijah loving to pull this one, going in disguise as a beggar or traveler of some sort. Treat him nice and you'll be rewarded umpteen times over. The Aesop of all the stories is basically "be nice to strangers and help those in need". Good Aesop.
According to some traditions, be became Sandalphon, an angel associated with prayer and children.
The Qashmallim, from Promethean The Created roleplaying game line. Interesting in that these entities, while unquestionably powerful and apparently representatives of a higher power, might not be actual angels as such. Their presentation, however, is very much angelic (often in the more inhuman "wheels with wings and eyes" vein).
In Warhammer both the Brettonian Green Knight and Grombrindal, the White Dwarf could possibly be this; the Green Knight may be the founder of Brettonia, while there are a lot of theories on the White Dwarf, again including the possibility that he's the founder of the Dwarven nation.
And in 40k, The God Emperor loved doing this to people.
In An Inspector Calls it is neavily implied that the inspector is not what he seems, although it is not explicit.
In Sonic Unleashed, Sonic's amnesiac new friend Chip, is revealed to be Light Gaia, the opposite of the final boss.
At the end of The Granstream Saga, Arcia turns out to have been a celestial being all along. Considering that this character is also one of the worst Purity Sues in video game history, this realization probably drove her past the point of likability for many players.
Joshua displays an assortment of clearly divine powers before later being revealed as the Composer.
And then, once you start getting the secret reports, you learn that Mr. H is literally an Angel. As well as Joshua's boss. Wings and everything, in the secret ending.
Red Dead Redemption contains the Stranger side-missions "I Know You" where a well dressed stranger from John's past meets up with him three times in each of the game's main locations. (New Austin, Nuevo Paraíso, and West Elizabeth/)The final meeting is the most notable, as John, finally fed up with not getting any answers from this guy, shoots him three times as he's walking away. The bullets apparently pass right through the guy, but leave no evidence that they even touched him. When John looks away for a moment, he's gone.
The man calls the last place that they meet "a fine spot." That same area is where John, his wife, and Uncle are eventually buried.
John: Damn you!
The Strange Man: Yes, many have.
The Interactive Fiction game Vespers begins with the Biblical quote, and begins as the characters, Medieval monks, grant a place to stay to an almost-dead, beautiful girl that arrives at their gates. It's a subversion: she's a demon.
It is very heavily implied (though not quite confirmed,) that the gravedigger in God Of War is Zeus or one of the others gods in disguise.
Subverted in the Adventure Time episode "Freak City". Finn secretly hopes a beggar is a disguised wizard or elf, and gives him food. The beggar reveals himself as "Magic Man" and rewards Finn with "a mystical magical favor" — turning him into a giant foot.