William Ellery Channing was a minister at the Federal Street Church in Boston during the early part of the 19th century.  Many of his sermons shaped and defined the "Unitarian Controversy" and are therefore considered definitive of American Unitarianism.

Biography

"The System of Exclusion and Denunciation in Religion Considered" (1815) - A discourse affirming that character, rather than doctrine, is the defining mark of Christianity and that the condemnation and exclusion of those with Unitarian beliefs from Christian fellowship is a form of persecution.   

"A Letter on Creeds" - (excerpt) This piece is a classic work that lays out the perils of creeds - how they separate us from the mystery and wonder of our religious search, how they are a "means of fastening chains on men's minds".

"War" (1816) - An affirmation of Christian pacifism. Delivered before the Congregational ministers of Massachusettes (the first discourse with this title).

"Unitarian Christianity" (1819) - (Also known as the "Baltimore Sermon")
Channing's "Unitarian Christianity" was an address composed with an acute awareness of its potential historical significance.  That it lived up to that potential, to be the single most important sermon in Unitarian history, seems to have been confirmed by historical consensus.  John White Chadwick,  writing in 1903, noted that the sermon "is agreed to have been the strongest ever preached by Channing on distinctly Unitarian lines, his most important contribution to the Unitarian controversy, and to the definite integration of the Unitarian body." It was this integrating effect, this function of the sermon as an organizational call-to-arms, that more than anything has secured its historical place.  It was not only a theological statement, but a basis upon which to act.  In Conrad Wright's apt phrase, it was a "party proclamation." Both the time of the sermon, and the occasion which prompted it, gave it significance.  In 1819 the theological battle between the Calvinists and the liberals had reached its greatest intensity, and the liberals were beginning to assert  their independence, and to accept, however reluctantly, the label "Unitarian."   Channing's sermon was the signal that the name had not only been accepted, but seized and affirmed.  Moreover, Channing's preaching the sermon at a new liberal church outside Boston signaled the intention of the Unitarians to make theirs a national movement.  Six years later, in 1825, the American Unitarian Association would be founded.

From the preface to the sermon from the book "William Ellery Channing: Selected Writings (Sources of American Spirituality)" edited by David Robinson

"Love to Christ" (Part One) - A consideration of the nature of love to Christ and its grounds. 

"Love to Christ" (Part Two) - A discussion of errors concerning love to Christ in the Christian world.

"Likeness to God" (1828) - A discourse at the ordination of the Rev. F.A. Farley, Providence, R.I.  Here Channing asserts that true religion consists in becoming more and more like the Supreme Being. Religious instruction should aim chiefly to "turn men's aspirations and efforts to that perfection of the soul, which constitutes it a bright image of God."

Introductory Remarks to the Works of W. E. Channing, D.D. (1841) - When his sermons were collected into a volume, Channing wrote this introduction.