The Huxley File is created by Charles Blinderman and David Joyce of Clark University and contains extensive essays and letters, photographs, and commentaries on T. H. Huxley's works including his Collected Essays, Scientific Memoirs, and several other works.
Mitchell, P. Chalmers. Thomas Henry Huxley: a sketch of his life and work (New York: G. P. Putnam's Sons, 1900) is a Project Gutenberg edition of Chalmers' biography of Huxley.
Project Gutenberg Titles by Thomas Henry Huxley provides links to over forty of Huxley's works including essays, autobiography, letters, and science texts are provided in text form.
Thomas Henry Huxley is the Wikipedia's entry summarizing his life and works.