I have been using LibraryThing for over a year and IMO it is a fantastic resource.  You can sign up for free and catalog up to 250 books.  If you want to catalog more than that you pay a one-time fee of $10, if I remember correctly.  For me, the great thing about it is being able to find books that I might never have read before.  It gives you a list of authors or books that you might like based on your library.  You can browse other peoples libraries (as long as they are public.  some keep theirs private) and see what they read and they also have quite a few message boards for people that like sci-fi, fantasy, non-fiction, etc.

I havent cataloged my D&D collection (dont see the point really) but pretty much any book with an ISBN number will be recognized in some form or fashion.  Older books without an ISBN may not show up in their database but I havent had a problem with that yet.

I highly recommend it.

Edit: I guess it has been a while longer since I signed up.  It is $10 per year or $25 for lifetime membership.  I did the lifetime membership.  Seemed like a no-brainer to me.  

I've been a member of Library Thing for a while now, and I highly recommend it for anyone who wants to catalogue thier books.  The resources that they have available are very useful, and more are being added all the time.  As has been mentioned, you can even view the libraries of others on the site and find people whose interests parallel your own (you can also opt to keep your library private, and then no one can view it but you).  The site also maintains various message boards for those who want to interact with others who collect/love books.  Well worth the minimal investment, especially if you have a large number of books of which you wish to keep track.

Mercenary_Roadie wrote:I check the LibraryThing out and find it to be a good source , but I don't like the fact that you can't put specific information about your copy of the book in it.

As some examples, you can identify whether you have a Hardback, softback or Mas-market Paperback version. You can't also identify the edition/printing of the book.

I am going to continue to play with it as it seems to be a great source for new reading material.

Books are identified whether they are hardcover or softcover based on the ISBN number.  So if you add a book using it, it should indicate which one you have.  It also shows printing and publishing information as well.  You just have to click on the title of the book to get to the book details.  From there you can also change the cover art to match the book you have.  But if you add the book using the ISBN you shouldnt have to bother with that.

Mercenary_Roadie wrote:aia,

I just wanted to let you know that I now HATE you with all of my heart.  
Because of you, I have been spending hours on the LibraryThing.

...

...gh...  

would you like other interesting websites?