LibraryThing is a site that allows you to keep a catalogue of all your books...I've periodically tried (through the years) to make a list of all my books, but I never managed to find an optimal solution...Then when I discovered LibraryThing a few years ago (before several other similar sites were created) I immediately decided it was exactly what I needed, and started to write my list there!
My profile is at http://www.librarything.com/profile/rossiele , and you can see statistics following the links in the page .
The result has been only a partial success... Several years and only 523 books later, I'm still very far from completing my catalogue. It was (and is) easy as far as English books are concerned, as al the information can be derived automatically from Amazon or some other source, but for Italian books usualy the only valid alternative in manual entry, which is extremely tedious (especially when you have to scan the cover, which happens with practically all my Italian books).
So I periodically fall off the wagon and stay months without adding anything, then I try to catch up, etc... Eventualy, sooner or later (much later, probably :-) ) I'll manage to complete this work, as I always add new purchases as soon as I can ; the problem is with adding old books, but little by little, who knows...
Anyway, I love LibraryThing because it gives you lots of statistics about your books! How many other people own them, how popular books are, who has libraries that have lots of books in common with you, etc...
As only a part of my library is there, data are only partial but I like all the same to check them, just out of curiosity... It's also interesting to look at other people's book collection, as those that have similar tastes very often have books I would find interesting too..One never ends discovering new things!
And for some books it's surprising to see how few copies there are among the more than 1,300,000 members... Only 6 of them for example have "The harbours of England" by John Ruskin, and only few more (10 people included me!) have a copy of his "Time and tide"... On the other hand, more than 50,000 people own any book of the Harry Potter series (which isn't a great data anyway, considering we are talking of more than one million people... I would tend to think that a lot more people would have at least one book of the saga)
I'm rather curious about the "median/mean book obscurity" indicator, which is 66/2,427 for my collection...But I have no idea whether it's high or low ! I don't think it's high, as I seem to have a lot of books nobody else has, in no language or edition... But I don't know whether it's an unusual value or not (and at any rate it won't mean anything until I've finished loading my books!)