It's a library thing, or perhaps a librarian thing, but I'm coming out of the closet - I am a fully addicted Librarythinger. Librarything.com is a revolution. I started using it in 2007 in order to have an online list of all the books that I'd ever owned or read. Whilst I'm no longer adding to my catalogue, I still use Librarything a lot for a variety of different purposes.
If someone recommends a book to me, I don't go to amazon.com to read a synopsis; I go straight to Librarything.com. I can read member reviews and look at the user generated tags for the book to gain an idea of both the content of the book and the general reaction to it. For example, if the tag 'tbr' (to be read) is heavily used on a book title, I want to know why. It would make me question things further. The groups on Librarything are also of interest and the grouping of all current information onto a homepage is a fantastic and relatively new feature.
I like the recommendations. I also like the concept of reconstructing the libraries of dead authors. I like that some libraries are using the technology to further the reach of the traditonal catalogue. However, this relies on student engagement, which is often the hardest thing to generate. Tags only become really relevant when a great majority of people join in, but they are also often personal to that individual and have little meaning in the wider community. I love Librarything.com. I spent an age cataloguing all my books, tagging them, reviewing them; but even I duplicated tags, confused terms, ran out of steam with the reviewing. I really like this tool but I am unsure as to its usefulness within the library setting.