I often visit gigapedia when looking for books. Although a google search with appropriate keywords will get you links for downloads, gigapedia provides a one-stop search. I don't know why gigapedia is moving (litigation? piracy issues?), and I'm sad that the website is moving.
Gigapedia is moving to a new portal http://library.nu/, and it provides the same functionality. There is no registration yet, so you can freely access its search engine without having to log-in. I've tried it, and it operates like gigapedia. For example, putting in "Gravitation" in the search field gives a list of books in library.nu pages; each page in turn contains more information about the listed book and links for download.
For people in developing (what a crazy description! developing?) countries, gigapedia has been a boon to book-lovers. Most students in developing countries find books unaffordable. As an example, a copy of Resnick, Halliday, and Walker would probably cost a fifth of the minimum monthly wage. A lot of us really care about the books, and we would be willing to buy them if they cost less.
Libraries do not provide enough here-- when budgets are cut, new acquisitions to public libraries are the first to go. (Our local officials would rather construct basketball courts and hold beauty pageants than subsidize libraries. ) For now, gigapedia's successor will definitely fill a need here in our islands.
update: you're now required to register before you can perform a search. The registration is easy enough. You are only asked for your email address, and once you get the activation email, you're ready to go.