Project Gutenberg - Over 30,000 titles available for download for free to your PC, Kindle, Sony eReader, iPhone, or other device.
Digital Book Index - Downloadable eBooks. Most free; some cost.
Infomotions - This is a collection of public domain and open access documents with a focus on American and English literature as well as Western philosophy.
Google Books - It's google; lots of great free stuff!
Electronic Text Center