I'm an avid reader. And the older I get, the harder it becomes for me to remember every book I've read. At the same time, I want to get recommendations from sources other than Amazon: friends, people who share my tastes, etc.
Shelfari is a free service that lets you build a virtual bookshelf of stuff you've read, see what your friends are reading, discover popular titles in specfic genres, and join discussion groups.
In other words, at the risk of overusing the metaphor, it's kind of like Facebook for bibliophiles.
After signing up for Shelfari, you can browse or search its library to find books to add to your virtual shelf. For any book you choose, you have the option of rating, tagging, and/or reviewing it. You can also mark it as something you've read, are reading, or are planning to read. All this requires just a few easy clicks.
Shelfari is also heavy on community features, stuff like which books got the highest ratings and most comments for the day, members who added the same books as you, and group categories ranging from Authors & Writing to World Literature & Culture.
Of course, Shelfari is by no means the only book-lovers site of its kind. Another popular destination is Goodreads, though I find that site's interface much less intuitive and attractive.
I do wish Shelfari offered some kind of integration with Facebook and/or Twitter. Even so, it's a great destination for anyone who loves books. If you want to "friend" me on the service, look for user justrick.