Many of them started out as Congregational churches in the early 19th Century. Denominations originally included United Church of Christ, Unitarian, Presbyterian, Methodist, Baptist (there s a spectacular one in Providence). Nowadays some congregations have died out and other more vibrant ones have bought the buildings so you might find an Anglican, a Baptist, a Korean Methodist, etc.
Most are the United Church of Christ (Congregational). Probably the 2nd largest number, but much, much less, are Unitarian-Universalist. Both sects that accept same sex marriage, by the way.
They could be of almost any Protestant denomination
Traditionally, most of these were New England Congregational Churches.