I recently stumbled across LibraryThing. I've been meaning to post about it for several days now, but can't stop playing with it long enough.
I'll call LibraryThing a 'community bookcase'. You can enter in your personal cache of books, tag them according to what they are, etc, review them. You can then see many things: who else has them, what other's they have, how many have it, what they thought of it... the list goes on. I keep finding new things to play with. For a bibliophile, this site is it.
Books are ridculously easy to enter. Take a stab at the title or author, and up comes a list. It pulls catalogs from Amazon, the Library of Congress, and others. I haven't found a book yet that it didn't know about. It will list every known edition and let you pick which one you have from a list, and even show you a picture of the cover if it has one.
For instance, I wanted to add 'The Gunslinger' by Stephen King. That's a book with many editions. I started by just trying 'The Gunslinger". That was close. "The Gunslinger King" narrowed it down nicely. I was down to six choices. I have the Trade Paperback edition, which I picked from the list and added. Now I have it in my library, including ISBN and other info, and it was that easy. Now I can go in and enter acquired dates, started and completed dates, etc.
And it's as close to free as you can get. Enter 200 books for free, or as many as you like for $10 (year) or $25 (life).
The real fun is in seeing what everyone else has. There is a zeitgeist page that lists catalogs by several fascinating metrics : most books in library, most popular books, most popular author, etc. I can browse other's who choose to make thier library public. I can watch what they're reading and be notified if they post something new.
I can export and import data, and even add a blog widget to display a random book from my collection.
The zeitgeist page speaks volumes about who uses the service. Science Fiction and Fantasy are far more popular than anything else. The top six books are all by one author (I'll let you guess, but tell you that author has written only six books. Got it?), and we don't find a non-SF/F book until number nine.
I'm just endlessly fascinated by this thing. It creates an intersting community of readers, one that I can't wait to delve deeper into.