Grammar Girl
Your friendly guide to the world of grammar, punctuation, usage, and fun developments in the English language.
The Online Writing Lab (OWL) at Purdue University is a semi-open source writing resource where you can look up the specifics of writing and access instructional material as a free service of the Writing Lab at Purdue University.
From the Academy of American Poets, poets.org has thousands of poems, essays, biographies, weekly features, all on-line and free!
Project Gutenberg is a volunteer effort to digitize and archive cultural works, to "encourage the creation and distribution of eBooks".