I have been using LibraryThing for a while now and I am quite happy with it.
Originally it was mostly a data-entry chore, but I bought one of their $15(USD) CueCat bar-code scanner and started scanning in my books (work in progress). The downside of having all these books on the site, I noted, is that while there is no longer a nightmare for data-entry, tagging become a bit of an issue, as I still have to type them in one by one (another work in progress).
Today, I discovered a very innovative feature called tag mirror which I have never noticed before. As according to the site, it "is a new twist to tagging. Rather than showing a member's tags, this shows what other members think of a member's books."
Here's a screenshot of it:
Until I have time to properly tag all my books, it definitely helps me analyze what my true interests are better than my own tags :)
Also see LibraryThing's blog post about their innovation.