There is a large amount of overlap between these collections as they are mostly based on one or two early large public domain e-text projects. Still, all are worth investigating. Project Gutenberg is one of the original public domain electronic text collections. Their materials are in plain text format to ensure that they will work on as many computer types as possible. On-Line Libraries NetLibrary's Free Reading Room has over 4000 titles, mostly public domain classics, nicely formatted.
Free registration is required.
They also sell downloadable e-books. All items are selected and cataloged by actual librarians. The University of Virgina's Electronic Text Center has over 51,000 titles in several languages. Many are for UVA users only, but there are thousands for the general public as well. University of Pennsylvania On-Line Books Page links to over 12,000 items.