The Decameron is an Italian tale, in which ten friends flee to the countryside to escape the Black Death, and spend their time telling each other stories--each telling a story each of the ten days of the book. Shakespeare turned the ninth tale of the third day into All's Well That Ends Well.
This translation of Plutarch's Lives, as this is often known, served as the basis for Shakespeare's Julius Caesar, Coriolanus, and Antony and Cleopatra. When writing Antony and Cleopatra, Shakespeare lifted entire speeches from this work.
Shakespeare copied Ovid avidly. Almost all of his allusions to Classical myth are believed to be based in Ovid, and the plots of many plays, including Romeo and Juliet, A Midsummer Night's Dream, Titus Andronicus, The Tempest, and The Comedy of Errors, are based on Ovid's poetry. Shakespeare's debt to Ovid, however, is most obvious in his poetry--the Rape of Lucrece and Venus and Adonis, his narrative poems, along with his Sonnets.