In Dan Abnett's Gaunt's Ghosts novel First & Only, Gaunt received his father's ring from his father's commanding officer, when he was orphaned. Later, he uses it for its security codes.
Piers Anthony's Incarnations of Immortality has a few, most notable the living ring Sning, who's passed around quite a lot, down to Orlene (who gives it to her lover Norton, before he becomes the incarnation of Time). The catch is, Orlene's not an orphan (though she thinks otherwise), both her parents are Immortal Incarnations. War and Nature respectively. And her grandmother is Fate.
Though in a surprising move, Butcher actually managed to make an Orphan's Plot Trinket...well, plot-relevant. In Fool Moon, Harry realized at the last second that said pendant was inherited silver, the one thing that could kill a loup-garou. Probably the most badass thing any Orphan's Plot Trinket has ever done.
It's also used in Blood Rites when Thomas uses his pendant to prove they're half-brothers.
And again in Changes When it turns out to also double as half of a map of the nigh-unmappable Never-Never.
In Jim Butcher's other series, Codex Alera, protagonist orphan Tavi has one even though he doesn't know it. His guardian Isana keeps a ring that had belonged to his father on a chain around her neck. Tavi grew up thinking Isana was his aunt, but in fact she was his mother, and the ring belonged to his father, the dead prince.
Oliver Twist's locket, which belonged to his mother Agnes and was the proof of his identity. Sally the nurse stole it, then she gave it to Bumble's wife, and then she and Bumble gave it to Oliver's half-brother Edward Leeford aka Monks... who threw it into the Thames to ruin Oliver's chance to inherit the fortune of their father. It didn't work, since Nancy and Mr. Brownlow still managed to help Oliver.
Subverted in Esther Forbes's Johnny Tremain — Johnny knows what the Orphan's Plot Trinket does, but when he tries to use it to reconnect with his relatives, they refuse to see him. They change their minds eventually.
Subverted in Terry Pratchett's Feet of Clay. Nobby Nobbs, who comes from a poor family, has a shiny, golden ring, and could be a descendant of the throne to Ankh-Morpork. It later turns out that it is all a cunning plan, and the ring (and other valuable items he owns) were probably stolen by the countless generation of thieving Nobbses.
Well, except that at the end of the book he mentions he has several other similar trinkets.
Also, Carrot Ironfoundersson is a) an orphan, b) has an old sword (beat up and completely nonmagical, but by Discworld logic this makes it an Infinity Plus One Weapon), and c) has an almost magical aura of leadership, but d) is not even slightly interested in being King.
Of course, given The Machiavellian Patrician, claiming to be the true heir is a good way to end up dead. However, the Patrician has fulfilled some rather major... requests for him over the years.
Considering all these necklaces, it's only fitting that the cruelest twist on the trope should come in locket form. It fits the letter of the trope exactly, but the spirit is a different matter altogether. Slytherin's Locket belonged to Voldemort's mother, Merope Gaunt, who sold it for a few galleons while pregnant, and which her son later stole and turned into one of his Horcruxes. Voldemort also used to "collect" (that is, steal, after harming or killing their owners) "trophies," some of which were later turned into Horcruxes as well.
In The Tombs of Atuan, Ged finds the MacGuffin on an island inhabited only by an orphaned brother and sister who turn out to be of royal descent.
Diane Setterfield's The Thirteenth Tale features a lovely aversion. An orphan is found with a page of Jane Eyre in his clutches... But the page is only barely peripherally relevant and offers no clue at all to his origins.
Lloyd Alexander's The Chronicles of Prydain contain an interesting subversion. Orphaned Princess Eilonwy has only one thing that belonged to her mother's family - her "bauble," later revealed to be the Golden Pelydryn, an artifact of great power. To her, it's just a glowing ball that she's played with since she was little. The subversion comes in the fact that Eilonwy has always known that she's descended from the House of Llyr, and the Golden Pelydryn doesn't allow her to find her family; it does, however, grant her access to her magical heritage until she willingly gives up that access to save her friends.
In Gene Stratton-Porter's Freckles, Freckles himself thinks very little of the clothing on him as a Doorstop Baby, but Angel is quite certain that it will reveal how much his mother loved him. He's adequately convinced that when she returns with them, he asks whether his mother loved him.
Mothers who love and want their babies don't buy little rough, ready-made things, and they don't run up what they make on an old sewing machine. They make fine seams, and tucks, and put on lace and trimming by hand. They sit and stitch, and stitch—little, even stitches, every one just as careful. Their eyes shine and their faces glow. When they have to quit to do something else, they look sorry, and fold up their work so particularly. There isn't much worth knowing about your mother that those little clothes won't tell. I can see her putting the little stitches into them and smiling with shining eyes over your coming. Freckles, I'll wager you a dollar those little clothes of yours are just alive with the dearest, tiny handmade stitches.
The necklace Tash's mother gave her before Galaxy of Fear is not particularly plot important, but looking at it reminds her of the love she had for her parents, and that convinces her that she isn't a clone - all the Tash-clones have different emotional responses to her memories.
The Sword Bearer has a string with a locket and a ring, from John Wilson's mother and father respectively.
The title item serves as this for the heroine in The Keepsake Ring by Helen Fern Daringer.
The elaborate baby clothes Margaret Thursday wore when she was found as an abandoned baby in Thursday's Child by Noel Streatfeild. Someone also left money for her keep every year.
Posy's ballet slippers in Ballet Shoes by Noel Streatfeild.
Vin's earing in Brandon Sanderson's original Mistborn trilogy. A two-in-one orphan trinket, as it is a reminder of the brother who left her, and the only token of her dead mother. And a three-in-one plot trinket as it is a Hemalurgic spike which grants her the unique ability to pierce copper clouds, the means by which Ruin can communicate with her, and the thing preventing her from tapping the power of the mists.
Laxdæla saga: Before Olaf departs for Ireland, his mother Melkorka, formerly a slave, gives him a golden arm ring which she got from her father Myrkjartan as a child. In Ireland, King Myrkjartan recognizes the ring and is therefore convinced that Olaf's tale is true and that he is really his grandson.
Robin's pedant in The Girl from the Miracles District. Apart from granting him a magical Deflector Shield, it's also the only thing his parents have left him with when they dropped him off at the Order's doorstep. Only not really; he's way too old for this to be his true backstory, and the pedant is the only thing he has left after the Order wiped out his memories.
Hinterlands book 2 of The Godslayer Chronicles by James Clemens features an orphan girl named Dart who has a mysterious invisible demon dog that followers her around. In a twist of the trope Dart ends up being the plot trinket. As the daughter of 2 Rogue Gods, she is the only complete God left in existence. As such only her blood is capable of empowering the titular Godslayer sword. And if the author ever gets around to writing the 3rd book her importance towards the future of the Gods may become even greater.