+1 to Project Gutenberg.
Do not ignore Project Gutenberg Australia,
http://gutenberg.net.au , a separate site, which due to differences in copyright law has a lot of material not available at the main Project Gutenberg--notably George Orwell's works.
To search for sources outside Project Gutenberg, the best resource I know is the "Online Books Page,"
http://onlinebooks.library.upenn.edu
Manybooks.net has a lot of material available elsewhere that has been conveniently converted into native eBook formats.
A caution on searches for "free eBooks." There are a host of spammy commercial sites that are just posting Project Gutenberg titles surrounded by tons of ads. And there are also pirate websites that might be a good place to pick up malware.