These are some of the sites I've got bookmarked on my computer. Some are more useful than others, and you'll find the same books coming up over and again. In a lot of cases you have to do the conversion yourself. Apologies if I'm repeating sites that other people have already listed.
Alex Catalogue of Electronic Texts
University of Virginia
Blackwells Online Bookshop - ebook section
eBooksLib.com
Arthur's Classic Novels
FreeBook.co.uk
HTI Modern English Collection
Online Library of Literature Penguin UK - ebooks
Planet Ebook
University of Adelaide
bookreporter.com
I'm also a keen reader in foreign languages, and have found some etext sites in Russian, Czech, French, German and Italian. I'll let you have a list when I get time.