What are the names of some of the major sites where one can order books online?

There are many websites that have books online. Here are some of them:

Amazon

google Books

Project Gutenberg

Barnes and Noble

Online Books (UPenn)

Some of these sites have free books (Project Gutenberg and Online Books). Some of the sites have both free and limited editions, so some of the books are completely free, but with others you can read some of the book, but have to pay to read the complete work (google Books). Others are just places where you can buy books online (Amazon, Barnes and Noble).