My new favourite website - LibraryThing.com
A while back I was looking for a website to park the list of ISBNs I'd got from the Bucks County Council website; the list being the books I'd borrowed over the previous couple of years. (It wasn't easy to get that list, however with a bit of screen scraping I managed.)
The websites I found seemed to be orientated towards keeping lists of books you owned, whereas I wanted to concentrate on books I'd borrowed from the library. The other day I came across librarything.com (linked from the Council's website in fact) where I could upload that list of ISBNs and it would look them up and pull out the cover picture and other information. One thing it wouldn't upload is the date I read them, so I've just finished filling that in for over 100 books.
I love this site. It seems that everywhere you look you can find new things - you can how many other people have got/read your books, you can see ratings, reviews, see who likes similar authors to you etc. etc.