The Balance card from a Deck of Many Things (DMG, p. 162) causes your alignment to change. Lawful becomes chaotic, good becomes evil, and vice versa.
A sentient magic item (DMG, p. 216) has its own alignment. It is not exactly changing the alignment of the wielder, but if the wielder behaves contrary to its alignment, the item can try to take control of the wielder, and effectively make them behave according to the item's alignment.
The Book of Vile Darkness (DMG, p. 223) changes the alignment to neutral evil of a non-evil creature that attunes to it and fails a DC 17 Charisma saving throw.
The Hand of Vecna and Eye of Vecna (DMG, p. 224) each change your alignment to neutral evil when you attune to them.
The Sword of Zariel (Baldur's Gate: Descent into Avernus, p. 226) turns your alignment to Lawful Good once you attune to it.
An Archway in the Tomb of Horrors (Tales of the Yawning Portal, p. 219) reverses your sex and alignment "by a powerful magic" when you pass through it.
A Magic Mirror in Curse of Strahd (p. 108) corrupts those who use it to do evil. A creature that uses it to summon an assassin to commit murder has a cumulative 25 percent chance that their alignment shifts to neutral evil.
An Amber sarcophagus in the same module (p. 191) grants dark gifts. Each time a non-evil creature accepts a dark gift, it must make a DC 12 Charisma saving throw or the creature's alignment changes to evil.
A character that becomes a vampire (MM p. 295) has their alignment become lawful evil. (A character that becomes a Lycanthrope becomes evil only if they embrace their new nature, so it is not forced; a character that becomes a lich will have an evil alignment, but choosing to become a lich likely requires you to be evil to begin with, so it is not forced either).
Visiting an outer plane. A plane's influence can affect visitors in various ways, including shifting their alignment to more closely match that of the plane (p. 59 DMG):
In Bytopia (DMG p. 59f), optionally at the end of each long rest taken on this plane, a visitor that is neither lawful good nor neutral good must make a DC 10 Wisdom saving throw. On a failed save, the creature's alignment changes to lawful good or neutral good (whichever is closer to the creature's current alignment)
In the Abyss (p. 62), optionally a non-evil visitor that finishes a long rest must make a DC 10 Charisma saving throw or become corrupted. If a corrupted creature doesn't leave the plane within 1d4 + 2 days, its alignment changes to chaotic evil.
In the Nine Hells (p. 64) optionally, at the end of each long rest taken on this plane, a visitor that isn't evil must make a DC 10 Wisdom saving throw. On a failed save, the creature's alignment changes to lawful evil. (Also mentioned under Pervasive Evil on p. 79 of Baldur's Gate: Descent into Avernus.
In Mechanus (p. 66), optionally at the end of each long rest taken on this plane, a visitor that isn't lawful neutral must make a DC 10 Wisdom saving throw. On a failed save, the creature's alignment changes to lawful neutral.
Planar Alignment in Morte‘s Planar Parade (p. 6) adds that in the Outlands, if exposed to it over the years, the influence of a plane changes a creature's alignment to match the plane's alignment, as shown on the Outer Plane Alignments table. (Matching the alignments in the DMG but providing those for all other outer planes too; thanks to @Jack)
A wish spell, with the DM's OK, can possibly change a creature's alignment.
Modify Memory, maybe. While the spell says that "A modified memory doesn't necessarily affect how a creature behaves, particularly if the memory contradicts the creature's natural inclinations, alignment, or beliefs", it could possibly affect the creature's behavior, and if used repeatedly and over long periods of time, might be able to manipulate its view on the world enough to make it change its alignment. It's debatable whether this should be considered "forced", so I include this here just for completeness; your mileage may vary on this one.
