Parish registers online
Nearly 1 million Hertfordshire baptisms and marriages are included in the IGI at FamilySearch but not all parishes are included and some of the entries are duplications (you can see the coverage here).
There are about 800,000 baptisms, 300,000 marriages, 160,000 banns, and 600,000 burials in the Hertfordshire Collection at findmypast and you can also view digitised images of the registers and Bishop's Transcripts from which the entries have been taken. There are very few entries at Ancestry.
The volunteers of FreeREG have transcribed some of the registers for about 25 Hertfordshire parishes. There is currently no OPC project for Hertfordshire.
Although there is an index to wills at the Hertfordshire Archives and Locals Studies website it doesn't include the many Hertfordshire wills held by the Essex Record Office. These can be searched free at the Essex Ancestors site, and you can view them online if you buy a subscription.
Births, marriages and deaths
There are currently NO local indexes of births, marriages and deaths for Hertfordshire.
HALS holds censuses for Barkway, Charlton, Digswell, Essendon, Hitchin, Kimpton, Langley, Northchurch, Preston, Reed, Therfield, Woolmer Green (various years 1700-1838).
Key Hertfordshire resources
Hertfordshire Archives and Locals Studies (HALS) is based in Hertford; to consult original documents you will need a CARN ticket which you can collect on arrival provided you take appropriate identification. Some of the records at HALS have been indexed and can be searched at Hertfordshire Names Online - these include Marriages and Marriage Licences, Apprentices, Wills, and Crime & Punishment records. A recent addition to the online indexes are the burials at Hill End Hospital (formerly Hertfordshire County Asylum) between 1899 and 1948; some of the people buried there died at the nearby Cell Barnes hospital, which opened in 1933.
Chipping Barnet, East Barnet, and Totteridge have been part of the London Borough of Barnet since 1965; however their registers are held by HALS and included in the Hertfordshire collection at findmypast.
Hertfordshire Family History Society meets at Woolmer Green, near Knebworth. Records or indexes published in booklet form or as CD ROMs include Hertfordshire Quarter Sessions 1588-1619, Hertfordshire Militia Ballot Lists (the most complete set of any county), and various Poor Law records - there is a full list here. There are also a number of ongoing projects.