Cardenio

Cardenio is a play suppsed to have been written by William Shakespare, but lost to modern scholars. According to Wikipedia's page on Cardenio, it's "known to have been performed by the King's Men, a London theatre company, in 1613. It is believed to have been written by William Shakespeare, probably in collaboration with John Fletcher."

• Double Falshood; or, The Distrest Lovers. Wikipedia on Double Falshood: "Double Falshood is a play by Lewis Theobald, produced in 1727 and published in 1728, which he claimed to have based on three manuscripts dating from the time of the English Restoration of an unnamed lost play by Shakespeare."
• The Second Maiden's Tragedy. From the Wikipedia Cardenio entry: "In 1990, Charles Hamilton, a handwriting expert, after seeing a 1611 manuscript known as The Second Maiden's Tragedy, usually attributed to Thomas Middleton, identified it as the missing Cardenio with names changed."