Goodreads - basically an easy way to keep track of what you read, want to read, have read, etc. Can reate books, see reviews, join groups. It's like Facebook, but for books!
Bookbub - tons of fantastic deals of ebooks (including free ones), you can also sign up for the mailing list and get deals/free books emailed to you everyday. I have been signed up for like two years and been ignoring them for awhile.. but I've enjoyed my deals