Bookcase is my little app for tracking my book collection. It's a pretty simple program, but it keeps track of title, author, and 24 other properties for each book. I used an XML data file format, and printing uses an XSLT file for formatting, so I'm rather proud of all that. KDE makes all that pretty easy.
It has automatic ISBN validation, which I'm rather proud of as well. Multiple authors are supported, and automatic formatting of the author's name is done.