Well, this actually turned into a Top 20 List LOL
I've not checked all these out yet and they are in no particular order aside from the ones I'm currently involved with being at the top of the list.
1) PaperBack Swap is your source for swapping books online - Read...Swap...Save! (Free except for postage.) As you may remember I joined this site six months ago and I'm loving it. I'm cleaning out the shelves of books I'm just not going to read again and getting books I want to read. (Please use the link in the sidebar if you sign up or at least say PeachesNCream sent you.)
I'll always have a lot of books in my home because I am a book-a-holic (no, ifs, ands, or buts about it) and have been ever since my sister checked out The Black Stallion from the school library (don't think she even got around to reading it...crazy horse girl she was back then -- heck, still is, LOL).
2) Shelfari introduces readers to our global community of book lovers and encourages them to share their literary inclinations and passions with peers, friends, and total strangers (for now). I'm also on this one though not very active. I believe this is hosted by Amazon.
3) SwapTree is a place where you can trade books, music, dvds, or video games that you own for thousands of items that you want all for free (except for the price of postage).
4) LibraryThing is a home for your books. Enter what you’re reading or your whole library. It’s an easy, library-quality catalog. Consisting of a community of 600,000 book lovers. LibraryThing connects you to people who read what you do. (Available for a small yearly fee.)
5) Feel like sharing your reading experiences with people close to you? At Booksprouts, it's easy to start a club, join a club, invite your friends, choose a book, and discuss! It's free, so join already!
6) blippr is all about making all things media in your life better. blippr is much more than just a trendy set of letters. It's a really easy way to discover, talk about, and organize great apps, books, games, movies, and music. We figure that hey, why not make it easy to talk about and then find even more great stuff to talk about? Come in and give it a shot!
7) 22book is dedicated to the creating, sharing, and viewing of book lists. Start out by browsing some of the featured lists to the left and then open a free account and start creating lists of your own.
8) BookJetty is for readers who want to keep track of good books they want to read, to check book availability in local libraries (from over 300 libraries in 11 different countries), to know what friends are reading, view Google Books preview, Amazon rating and sales rank, bookshelf widget on my blog, and probably more.
9) BookGutton is a new way to read online and it's free.
10) GuruLib's mission is to help catalog, connect and share books, movies, music and games between home libraries.
11) Listal is a place to list, rate, tag, and review items. Share with others and discover new things to enjoy. A social network -- See what's popular among your friends or find people with similar tastes based on ratings. A user contributed database, including videos, pictures, reviews, lists, and more.
12) Goodreads is a free website for book lovers. Imagine it as a large library that you can wander through and see everyone's bookshelves, their reviews, and their ratings. You can also post your own reviews and catalog what you have read, are currently reading, and plan to read in the future. Don’t stop there – join a discussion group, start a book club, contact an author, and even post your own writing.
13) BookMooch is a community for exchanging used books. BookMooch lets you give away books you no longer need in exchange for books you really want. Every time you give someone a book, you earn a point and can get any book you want from anyone else at BookMooch. Once you've read a book, you can keep it forever or put it back into BookMooch for someone else, as you wish. There is no cost to join or use this web site (your only cost is mailing your books to others).
14) FetchBook - New & used books - Find the lowest price - Compare 140 book stores, 60,000 sellers, in a click.
15) Fantastic Fiction has Bibliographies for over 20,000 authors. And information on over 270,000 books.
16) BookCrossing is earth-friendly, and gives you a way to share your books, clear your shelves, and conserve precious resources at the same time. Through our own unique method of recycling reads, BookCrossers give life to books. A book registered on BookCrossing is ready for adventure.
17) BetterWorldBooks collects and sells books online to fund literacy initiatives worldwide. With more than two million new and used titles in stock, we’re a self-sustaining, triple-bottom-line company that creates social, economic and environmental value for all our stakeholders.
18) Rare Book Room has been constructed as an educational site intended to allow the visitor to examine and read some of the great books of the world.
19) Backstory is a blog where authors share secrets, truths, logical and illogical moments that sparked their fiction or memoirs.
20) Everyone is familiar with Amazon for New and Used books but I want to include Powell's which is a New and Used bookstore in the Pacific Northwest and I trust the used books from Powell's over Amazon every time.