Booksfree.com - rent books & audiobooks online and save 60-80% over buying them
NetGalley - delivers secure, digital galleys to professional readers. If you are a reviewer, blogger, journalist, librarian, bookseller, educator, or in the media, you can use NetGalley for FREE to read and request titles before they are published
Shelfari.com - a community-powered encyclopedia for book lovers
IndieBound.org - When you shop at an independently owned business, your entire community benefits
Goodreads.com - book recommendations & book clubs
I'm sure there are a lot more that can be added - do you know any?