An index (limited transcript) of Essex parish registers 1538 – 1900 are now online free at
https://familysearch.org/search/collection/show#uri=http://hr-search-api:8080/searchapi/search/collection/1465709
There are no images available at the familysearch site, but scans of the actual registers can be found at
http://seax.essexcc.gov.uk/displayParishContents.asp
And last, but not least the Essex Online Parish Clerk is at http://essex-opc.org.uk/
