Goodreads is a fantastic resource for keeping track of what you have read and plan to read, leaving reviews and seeing what others have said about books you are thinking of reading, and participating in a reading-loving community.
LibraryThing is an alternative to Goodreads. Whereas Goodreads is more of a social media site, LibraryThing has more critical reviews of books. The user interface is a bit harder to use and there is a fee for logging more than 200 books but for just reviews, it is a nice second perspective.
Serving the same general purpose as Goodreads (a 5-star rating system, reviews, lists), Letterboxd is aimed at the review of movies instead of books. Letterboxd can also connect to Twitter and Facebook, allowing users to view films mentioned by friends.
Book Finder searches AbeBooks and several other websites to find the best price possible for the book you need. My experience is that the best results usually come from AbeBooks, but double-checking on BookFinder could save you a few dollars from time to time.
Book Sale Finder, as the name implies, helps you find book sales. It covers the entire United States, with each state having its own dedicated page. On each page, you can find posted local sales sorted by date and city as well as lists of bookstores and ongoing sales.
Book Series In Order, as the title suggests, provides handy lists of books and series based on author name. It also lists them by series and provides information about each series as well as the individual books. With these features and more, this website is very handy for researching lists of books, reading order, and more.
Best Fantasy Books’ is a fairly detailed website that organizes fantasy books into subgenres, where users can up- or down-vote novels. The website is a fantastic resource for any reader interested in the fantasy genre and also has small sections for recommending science fiction books, movies, and games.
Bartleby is fairly similar to Project Gutenberg, being a database of free domain material. While their core features are similar, Bartleby can do a few things Gutenberg cannot such as searching for quotes. And while their database is smaller overall, they focus more on traditional literature and feature documents not available on Gutenberg, such as works in Latin.
Similar to the previous two sites listed above, PoemHunter is a database specializing exclusively in poetry. Along with works from well-know poets, the site also allows users to create an account and publish their own poetry.
LibriVox is a bit like the audiobook equivalent to Project Gutenberg. Their database contains thousands of free domain audiobooks, even featuring several hundred in other languages, and is constantly growing.
Formerly known as CourseSmart, VitalSource is an online resource for renting or purchasing e-textbooks. Their rental prices tend to be much cheaper than buying a physical book, so long as you do not mind reading from a screen.