Try these first:

Digital Book Index
Search or browse over 130,000 free online books by title keyword, author, or subject.  This index is intended as a "Meta-index" for most major eBook sites, along with thousands of smaller specialized sites.

Online Books Page
A listing of more than 25,000 works hosted by the University of Pennsylvania Libraries. You can browse by subject or call number and search by title or author. New books are added daily.  

Google Book Search
Google's book search engine includes both full and limited-access titles. Use the Advanced Search to specify words in title, author, date or publisher. The complete text of many public domain books (published before 1923) are included, as well as brief excerpts from other books.

Project Gutenberg
Claims to be the “first and largest single collection of free electronic books.”  Books are available in many languages and you can browse and search a variety of ways.

BookYards.com
Several options available to browse this collection of over 15,350 books.

Free-eBooks.net
Thousands of free ebooks available to download with free registration.

Manybooks.net
Over 16,000 ebooks available in a variety of formats, including for portable devices.

Netlibrary
Over 3,000 ebooks that are full text searchable.  You can also search by subject and publication year along with the regular title, author, and keyword search.  NYU-Poly login required.

Open Library
Created by the Internet Archive, this site offers access to important book collections from around the world through a visually appealing and easy to use interface.

Oxford Text Archive
Several thousand high-quality, well-documented electronic texts in a variety of languages. Its holdings include electronic editions of works by individual authors and standard reference works such as the Bible and dictionaries.

Text Archive
Part of the Internet Archive, this site includes a variety of sub-collections such as American Libraries and Biodiversity Heritage Library.  The Internet Archive also includes audio, moving images, software and archived web pages.

World Public Library
A consortia of 65 members offering over 75,000 free books available in the Public Access Section in HTML format.  No advanced search available.


Special Collections:

American Mathematical Society Books Online

Cornell University Library Historical Mathematics Monographs

Digital Library of Science

Digital Mathematics Library

E-DSP (Computing, Math and Engineering)

National Academies Press (search for "free" for freely available titles)