This is a fantastic resource! Let's hope that other counties provide something similar soon.
A slightly easier way to search for the parish you want:
Go to
http://seax.essexcc.gov.uk/displayParishContents.asp.
Choose the first letter of the parish you want, then select the parish and the name of the church.
A list of record types and dates will appear. If it says 'Digital Image' in the 'Format' column, you can view the images online.
Click on 'view' to start looking!
It seems that parishes beginning with A have the most coverage at the moment. Here's a rough list of parishes and dates.
Aberton, St AndrewBaptisms 1560-1971
Burials 1564-2002
Marriages 1564-1689, 1711-1950
Abbess Roding, St EdmundBaptisms 1560-2005
Burials 1567-2005
Marriages 1567-1753, 1756-1990
Aldersbrook, St GabrielBaptisms 1903-1962
Marriages 1914-1961
Aldham, St Margaret and St CatherineBaptisms 1559-1879
Burials 1564-1944
Marriages 1558-1984
Alphamstone, St BarnabusBaptisms 1705-1939
Burials 1705-1812
Marriages 1705-1755, 1814-1836
Alresford, St PeterBaptisms 1742-1997
Burials 1742-1992
Marriages 1743-1971
Althrorne, St AndrewBaptisms 1813-1980
Burials 1813-1951
Marriages 1800-1981
Ardleigh, St Mary the VirginBaptisms 1555-1967
Burials 1555-1643, 1661-2001
Marriages 1555-1642, 1662-1717, 1723-1963
Arkesden, St Mary the VirginBaptisms 1690-1875
Burials 1690-1915
Marriages 1690-1985
Ashdon, All SaintsBaptisms 1553-1553, 1558-1980
Burials 1553-1553, 1557-1609, 1646-1646, 1652-1670, 1675-1675, 1696-1980
Marriages 1557-1598, 1610-1669, 1696-1958
Asheldham, St LawrenceBaptisms 1813-1966
Burials 1813-1965
Marriages 1755-1964
Ashen, St Augustine of CanterburyBaptisms 1560-1906
Burials 1558-1812
Marriages 1558-1951
Ashingdon, St AndrewBaptisms 1566-1947
Burials 1564-1962
Marriages 1568-1753, 1761-1835, 1838-1971
Aveley, St MichaelBaptisms 1563-1622, 1718-1971
Burials 1571-1646, 1718-1939
Marriages 1720-1956
Aylthorpe Roding, St MaryBaptisms 1559-1644, 1666-1965
Burials 1560-1642, 1666-1992
Marriages 1560-1642, 1666-1751, 1755-1906