I am tempted to node a completely different Essex in a completely different part of the world, but (sigh) I must finish what I started.

The domain of the "East Saxons", Essex was a separate kingdom, part of the Anglo-Saxon Heptarchy. Essex never rose to prominence like Wessex or Mercia.

Anglo-saxon kings of Essex

Aescwine (527-587)
Sledda (587-604)
Saebehrt (604-616)
Saexred and Saeward (616-617)
Sigebehrt I (617-653)
Sigebehrt II (653-660)
Swithelm (660-665)
Sighere and Sebbi (665-683)
Sebbi (683-695)
Sigeheard and Swaefred (695-709)
Offa (of Mercia) (709)
Saelred (709-746)
Swithred (746-758)
Sigeric (758-798)
Sigered (798-825)

Egbert of Wessex annexed Essex in 825. It became part of the inheritance of supernumerary sons on two occasions. The rulers of Essex after 825 are detailed in the writeups for Kent and Wessex, and later with the Rulers of England. After Alfred the Great rescued Wessex from oblivion, Essex was part of the Danelaw while that entity existed.