LibraryThing is just one of several tools out there for bibliophiles to keep track of books they have read, share book titles they are currently reading, and keep track of (at least in my case) a massive to-be-read (TBR) list.
I use Shelfari, but that's only because it was the first one to which I was introduced.
Plus, I love the name! What is reading, if not a safari: 1. An overland expedition, especially one for hunting or exploring. 2. A journey or trip. Shelfari is a clever play on words and I love a clever play on words. But I digress.
My fabulous friend Deanne introduced me to Shelfari, and after entering 1,454 books to date, I am loathe to start over anywhere else. You can set reading goals (and get the very irritating message "you are behind your reading pace" as encouragement), join groups, see which books are popular, see which members are posting the most books/reviews/comments, see which readers are most like you (through a comparison of what you have recently read and are currently reading)...it's pretty good.
GoodReads is another tool that seems to be very popular. I like GoodReads because the summary of the book is usually a little more concise than that of Shelfari. But that's the only reason I go over there, normally. It is very similar to Shelfari, in which I am already invested.
And that brings us to Debbi's comment on LibraryThing. The immediate and coolest thing I note on this site is the Zeitgeist tab at the top. (NOTE: Zeitgeist: ideas and spirit of time: the ideas prevalent in a period and place, particularly as expressed in literature, philosophy, and religion). It is a compilation of the "top" of several categories. Top 25 most reviewed books. Top 25 works by rating. 50 largest libraries. Very cool stuff!
I am still poking around this one, but it seems to be about the same as my beloved Shelfari, with enough cool extras to make me bookmark it. Overall, these online tools are indispensable for the organization of many books, finding new books, and sharing thoughts and ideas with other bibliophiles across the continents. If you haven't already, check one of these out, or let me know if you use another one.