The Barbour Collection is the best early vital records collection for Connecticut birth, marriage, and death records, aside from town vital records. It’s named after Lucius B. Barbour, Connecticut’s examiner of public records in the early 1900s. It’s a statewide index of Connecticut birth, marriage, and death records listed alphabetically and by towns.
The date ranges vary by town, based on when the town was created and started keeping records. In Connecticut, by law, each town was and still is responsible for keeping and maintaining the birth records, marriage records, and death records for that town.
Barbour, as well as those he enlisted, went town to town copying these vital records. They attempted to compile records through 1850 but some towns have records up to 1870. The Barbour Collection is not complete, and AmericanAncestors (NEHGS) has a great article explaining some known deficiencies.

David Rumsey Map Collection 1855 Map of Connecticut published by Desilver & Butler Cowperthwait