An ongoing debate, this one. If we are to believe that Shakespeare didn't write all of the plays attributed to them - indeed, any, according to some theories - then Marlowe comes out as one of the top three candidates as author, the other two being Francis Bacon and Edward de Vere. As for proof, lots has been written. Try getting hold of a copy of 'The Story That Sonnets Tell' by A.D. Wraight - it's supposed to be one of the best ion the subject.