* "The Tragical History of Romeus and Juliet" by Arthur Brooke (1562): This long narrative poem, based on a 1530 Italian novella, is considered the most significant inspiration. It tells a story remarkably similar to Shakespeare's play, including the central romance, feud, and tragic ending. Scholars believe Shakespeare likely used Brooke's poem as a primary source.
* "The Palace of Pleasure" by William Painter (1567): Painter included a version of the Romeo and Juliet story in his collection of Italian tales, though it differed slightly from Brooke's work. It's possible Shakespeare was familiar with this version too.
* Other Classical and Medieval Literature: Shakespeare would have been familiar with the story of Pyramus and Thisbe, a Greek myth that also depicts a thwarted love due to a feud. He also likely drew inspiration from the idea of star-crossed lovers, a popular theme in Renaissance literature.
It's important to remember that Shakespeare was a master of reworking existing stories, and he often blended elements from various sources. While "Romeus and Juliet" is the most likely single inspiration, Shakespeare's own genius and creative vision transformed the source material into the timeless tragedy we know today.