There were a number of men who held the title Earl of Essex, but the most famous (or infamous)--and the man mentioned by William Shakespeare in several of his plays--was Robert Devereux, 2nd Earl of Essex (1565-1601). Convicted of treason following an unsuccessful coup d'etat, Devereux was the last man beheaded in the Tower of London in 1601. The 2nd Earl of Essex was referred to by Shakespeare in Henry V and was alluded to in Much Ado About Nothing. Devereux is also believed to be the basis for some of the dialogue in Hamlet.
A previous Earl of Essex, Thomas Cromwell, 1st Earl of Essex (1485-1540), was King Henry VIII's chief minister until he was also executed for treason in 1540. Cromwell appears in Shakespeare's Henry VIII.