LibraryThing

Oh wow! Now this is a very cool tool and that's not just the cataloger in me talking! You don't even have to own any books to get benefit from The LibraryThing! You could include books you've read but don't own. The tagging would be really helpful when you don't remember the title of a book but you remember the content.

It was a bit of a challenge getting the widget created. A combination of the comments by iStar and clarification from fourdogmom got me through it.