The Phantom of the Opera: In addition to the Phantom's distinctive chromatic chords, the phrases "He's there, the Phantom of the Opera" and "I am your Angel of Music" (which refer to opposing aspects of his personality) share the same melody.
Almost every song in Evita is a leitmotif of the singer:
For Eva herself, her songs are almost all based on the tune of "Don't Cry for Me Argentina", "I'd be surprisingly good for you", or "A New Argentina".
For Peron, it's most often "Dice are rolling"
The aristocracy and the army both have their own themes, first heard in "Peron's latest Flame", which are later repeated whenever they speak.
But the most chilling example is her dressers/cosmeticians. They first sing their theme in "Rainbow High", and then repeat it in "Lament"- only the latter scenario has them repeating the tune (and most of the lyrics), but said while preparing and preserving her corpse.
Older Than Radio: Richard Wagner's operas, particularly Ring of the Nibelung, are packed full of leitmotifs which represent each character or plot element. In fact, Wagner is regarded by many as the original creator of the Leitmotiv technique in composition, and though this is not absolutely accurate, he was certainly its popularizer and its most devoted and thorough practitioner.
Jekyll, in Frank Wildhorn's Jekyll & Hyde musical, has a dark piano and violin theme that plays when he sings in group songs, from the dramatic Board of Governers to the light-hearted Engagement Party.
Sondheim employs leitmotif in most of his major scores, but the technique is most evident in its truest sense in his Sweeney Todd: The Demon Barber of Fleet Street; melodies, accompaniment figures, rhythmic patterns and chords are introduced, fragmented, developed, reprised and interwoven throughout the entire score, each of them representing a different character, mood or aspect of the story. Perhaps the most awesome example is also the most spoiler-y: The Beggar Woman's Jig motif ("How'd you like to fish me squiff, dear?") is actually the minuet waltz heard at the Judge's ball in the song "Poor Thing" when the Judge prepares to rape Lucy, Sweeney's wife, which clues us in to the fact that the Beggar Woman is actually Lucy. Likewise, the Beggar Woman's "Alms, Alms" motif - based upon a falling second - is derived directly from the "Eleison" motif used to represent Sweeney's keening for his lost wife ("And my Lucy lies in ashes"). The two motifs pointedly merge together in the underscore at the very end of the play, when Sweeney finally makes the connection too late.
In Show Boat, non-singers Captain Andy and Parthy have characteristic themes, though Andy's is sung by the chorus a couple of times. Though Magnolia is a singing character, she also has an instrumental motif first heard when she plays it awkwardly on a piano.
You might not expect to find Wagnerian Leitmotifs in a fairy-tale opera for children. But Humperdinck's Hänsel und Gretel has quite a few.
Les Misérables has a ton of them. The Thenardiers' songs are almost entirely repeats of the melodies of "The Waltz of Treachery" and "Master of the House", and then "One Day More" is Valjean's main theme ("Who Am I?") combined with transposed versions of "I Dreamed a Dream", "Master of the House", and Javert's little leitmotif that's featured in "Fantine's Arrest" and "The Robbery" put into a major key. Oh, and not to mention "Look Down" is repeated 3 times in the show, and the song "Turning" is "Lovely Ladies" at a slower tempo.
The tritones used when someone is in trouble, like in "Lovely Ladies", "Rue Plumet" and "Castle on a Cloud"
For that matter, "Empty Chairs at Empty Tables" - Marius's song of mourning for his lost comrades - is the same piece of music that the Bishop of Digne sings in Act 1 when showing generosity to Valjean. Since true leitmotifs are meant to be associated with a specific character, mood or idea, it's uncertain to what extent the repetition of music in Les Misérables is actually leitmotif in action or just practical frugality on the part of the composer.
Not to mention "What Have I Done" and "Javert's Suicide" have the exact same tune too (except that a few lines in the latter are an octave lower), even some of the lines are the same or similar. Just, you know, the outcome is different.
"Who Am I?", Fantine's part of "Fantine's Arrest", "Come to Me (Fantine's Death)", and "The Finale" all have certain parts of "On My Own". Fantine also has a leitmotif in the latter three.
The Original French Album has a reprise of "Red and Black", but slower.
The Most Triumphant Example is probably what might be called the main theme of the musical. The theme is used over and over again, in "The Convicts," "The Runaway Cart," "The Confrontation," "The Beggars," and many others. It is overwhelmingly the theme of strife - the song plays over any scene of struggle or suffering - and as such, could be considered to define the musical.
In Bernstein's Candide, Cunegonde's theme first appears as the melody of "Candide's Lament" (originally a Cut Song but included in most later productions). The theme is subsequently transformed into the "Paris Waltz," an instrumental valse brilliante which is echoed at the climax of "You Were Dead, You Know." The first act finale has the theme sung in counterpoint with the melody of "My Love," and the theme plays a decisive role in the finale ultimo, "Make My Garden Grow."
In the musical RENT, there's a little musical theme that is played every time Collins come onstage, and it's a good part of the melody/background music in his song "Santa Fe." Also repeated when singing about the dead Angel later.
The most obvious example for RENT is arguably Roger's version of "Musetta's Waltz", which is composed for Mimi and played when she is thought to be dead.
Composer Michael John LaChiusa makes frequent use of motifs, especially in Hello Again (with characters echoing sentiments expressed by others), and in Marie Christine (in which an eerie tritone set to the word "beautiful" and "arrogance" is frequently repeated.)
Wagner's use of Leitmotif was likely suggested by the practice of Carl Maria von Weber, whose Der Freischütz Wagner adored. The figure featuring a diminished seventh and ominous timpani thuds that introduces the devil Samiel in that work is one of the most famous uses of Leitmotif in Early Romantic music.
Stravinsky's music for the ballet The Firebird is heavily leitmotif-laden. There are identifiable themes for the Firebird, the villain, the hero, the princess, and the villain's monster guardians.
Wicked has two major recurring motifs, the Overture (aka Elphaba's theme), and "Unlimited". The former is based on an earlier composition by Schwartz from The Survival of St. Joan. A lesser motif, also heard during the overture, is associated with the flying monkeys.
The Russians in Chess (excluding Anatoly) have one. It's first heard as the creeping oboe from the beginning of "U.S. Versus U.S.S.R.", and it returns with a vengeance for "Soviet Machine".
The opera Boris Godunov has a noble-sounding motif for Dimitri, the slain Tsarevich, which is appropriated by his impostor, the Pretender. Another motif represents Boris's guilty conscience, though it only appeared once as the opera was originally composed.
The Unsinkable Molly Brown has "Molly's Spinach Music," which consists of the theme of "I Ain't Down Yet" played on a muted trumpet "symbolic of rolling up her sleeves and reaching for the spinach can" (as the script puts it).
Benjamin Britten's Albert Herring has a few of these:
The First of May motif, first heard as part of Lady Billows's opening address, then as a horn call at the start of the second act and played on bells for Albert's coronation.