The Online Books Page
A good starting point, this index from the University of Pennsylvania covers over 20,000 freely available ebooks. Most titles are older, out-of-copyright editions, but a smattering of recent titles are available.
E-Scholarship Editions
This project from California Digital Libraries provides free public access to hundreds of current, academic titles from the University of California Press. (Choose the "public" collection; other titles are limited to the UC community.)
National Academies Press
Free access to over 3,000 current publications of the National Academies of Sciences; emphasis on health, science, technology and public policy.
Bartleby.com
This commercial site provides free web access to several current reference works as well as many older, public domains of reference works and "great works" such as the entire Harvard Classics collection.
Internet Public Library Book Collection
The IPL collection contains over 20,000 titles that can be browsed by author, by title, or by Dewey Decimal Classification. Titles and authors can also be searched by keyword.
Project Gutenberg
The internet's oldest producer of free electronic books. All the texts are talem from books no longer in copyright, usually pre-1923. You will find classic books from the start of this century and previous centuries.