A good (lengthy) article on the evolution of the American Unitarian
movement from Congregationalism is at the Unitarian Universalist
Association website.
    The Unitarian Controversy and Its Puritan Roots

A much shorter article (but one that goes back even further in time)
is part of the Columbia Electronic Encyclopedia.

A few excerpts from that:

    Unitarianism, in general, the form of Christianity that denies the
    doctrine of the Trinity, believing that God exists only in one person.

    Reason and conscience were considered the only guides to religious
    truth; complete religious toleration, innate human goodness, and
    universal salvation were preached. Unitarianism took hold in the
    liberal wing of the Congregational churches of New England.

    A congregational form of government prevails in the Unitarian
    churches, each congregation having control of its own affairs.
    Neither ministers nor members are required to make profession of any
    particular doctrine, and no creed has been adopted by the church.
    The covenant in general use is simply, "In the love of truth, and in the
    spirit of Jesus, we unite for worship of God and the service of man."

    Rick Parker

Peter Montgomery wrote:
> I'm pretty sure Congregationalism got in there some where.
> Unitarianism is a unique phenomenon: a noble attempt to
> have all good things all ways. The result is the gathering
> of good people with not so much in common. If one were
> looking for roots, by definition such limiting distinctions
> would not be found there. (That's my outside view, anyways.)

William Gray wrote earlier:
> Peter,
>     In my experience, Unitarianism (his family's religion) and
> Anglicanism (his adopted/conversion religion) are far different. I have
> visited both his family's Unitarian church in St. Louis and his Anglican
> church (St. Stephen's) in London and have studied a little about the two
> religions. Unitarians (and I mean no offense by this) have far fewer
> beliefs than Anglicans. I am no expert on either of these, but my
> understanding is that Unitarians are far more accepting of other
> religions within their own church, while Anglicanism tends to be more
> exclusive.
> Those friends of mine who are members of the St. Louis Unitarian church
> find it slightly offensive to think that Eliot converted, rather than
> reverted, but I think that's more accurate understanding.