Edward the Confessor: King of England, married to Harold's sister, Edith. He died in January 1066 without an heir.
King Cnut: King of England 1016-1035. Cnut was the King of Denmark, who exploited the fragmented nature of England to seize the throne in 1016. He ruled with the help of the English Earls Godwine and Leofric.
William of Normandy: Bastard son of Duke Richard II, Edward the Confessor's father-in-law. William had a very shaky claim to the English throne, but what he did have in his favour was a dukedom full of Norman knights, all eager for a share of newly conquered land.
Harold Godwinson: Son of Godwine and Earl of Wessex. Harold was very powerful by 1066. He was possibly richer than the King, and had established alliances with all the major magnates of England. He could claim only a tenuous link by marriage to the family of Cnut, but he was the brother-in-law of King Edward and despite having the weakest claim to the Crown, he was in the strongest position. William claimed that Harold had sworn an oath to deliver the Crown up to William on King Edward's death. This is probably a fiction.
Edwin and Morcar: Grandsons of Leofric, Earls of Mercia and Northumbria. Previously arch enemies of the Godwinsons, they seem to have made a deal with Harold in 1065, who helped Morcar into the Earldom of Northumbria in return for their support when Edward died.
Tostig: Brother of Harold and ex-Earl of Northumbria. Deposed by the Northumbrians in favour of Morcar, Tostig fled to Norway, where he plotted revenge against his brother Harold.
Harald Hardrada: King of Norway. Persuaded to invade Northumbria in 1066 by Tostig. Their victory at Fulford and their defeat and death at Stamford Bridge probably ensured the success of William's invasion at Hastings.
Waltheof: Earl of Huntingdon and rightful Earl of Northumbria. Waltheof was too young to take up the Earldom of Northumbria when his father died in 1055, so it went to Tostig. He was old enough for the Earldom in 1066, but it was given to Morcar. His subsequent actions after the Conquest can be interpreted (to a point) to be attempts at getting his Earldom back.
Edgar the Aetheling: Aetheling means 'throneworthy' and was the title given to the legitimate heir to the Crown. Edgar however, was too young in 1066, and nobody wanted an unstable regency.
Swegn Estrithson: King of Denmark. Arguably the most powerful of the contenders, Swegn could claim direct descent from King Cnut. However, he was distracted by his own kingdom, and it was not until he died that his second son, Cnut the Holy, concentrated on England.
Archbishops Stigand and Ealdred: Archbishops of Canterbury and York. Primates of England.