GoodReads.com

Yet another bookish site... goodreads.com -- this one very similar to LibraryThing, except it allows you to show off what you're currently reading (using "shelves" instead of "tags" that can, actually, function in the exact same way, except it does isolate things like "to read" and "currently reading"). Also, it doesn't require you to own the book for you to feel comfortable adding it to your library, which is a big plus for public library patrons.