Well, I've been a little too enthusiastic buying books this year.
I use "BookCook", which is a great (and cheap) programme. I used to have an excel list, but only with the book titles so I wouldn't buy a book twice, but then I heard about BookCook and immediately bought it. Here's a screenshot:
It's especially great since it not only shows you each single book, but shows lots of statistics with one click. I've only had it for six months, but I wouldn't want to miss it anymore.