Edit: BOY OH BOY I EFFED UP THOSE LINKS. Thanks to Aric (@firstname.lastname@example.org) for the heads up!
Before I converted much of my personal library to digital, I probably owned somewhere around 2,000 books. The way I kept track of most of them was a LibraryThing account. Once I tried them out, I gladly forked over the embarrassingly low price of $25 for a lifetime membership. Adding the books was easy, especially since I had a barcode scanner to help me.
But as my philosophy changed towards physical things, I became more and more of a minimalist and I doubt I have two hundred books in my home now. Looking at the shelves around me, yeah, I think it's somewhere between 100 and 200 volumes. I went for the eBook revolution pretty hardcore and don't need LibraryThing to track eContent anymore.
Still, that's just me. Lots of people really love their physical books and some of you may want to keep track of them. Well, as of today, if you want to keep track of them using LibraryThing, you can do so for the low, low price of free. Get yourself a free account, maybe snag an inexpensive barcode scanner, and build your library!
For those curious what I might be using to keep track of my eBook collection, allow me to introduce you to Calibre.