The long-suspected collaboration between William Shakespeare and Christopher Marlowe on Henry VI will be made official in the New Oxford Shakespeare. Scholars estimate that 2/5 of the Bard's plays involved a collaborator to some degree or another. Heck, there has even been serious speculation that Shakespeare collaborated on Hamlet Special Edition with some obscure Polish author.
For more on Shakespeare, Marlowe and the New Oxford Shakespeare, see here.*
For more on Hamlet Special Edition, see Wikibard and Stanley's Marvelous Comics.
* The author has taken perverse pleasure in not using the Oxford comma in this sentence.