I must be crazy to spend my precious free time on this, but I have recently become an editor to The Internet Speculative Fiction Database (ISFDB). My goal is to enter information about all my SF&F books into the database. I track my progress on this Wiki page.
I have toyed with the idea of doing something like this for a long time. In earlier times, when I still owned a Windows machine, I had my own MS Access database where I kept an inventory of the books I own. The database structure was even detailed enough so that I could add short stories as separate entities and assemble them into anthologies. The database project never progressed further than that, and after a while I even stopped keeping the data up to date. After all, I realized, the whole thing only served to satisfy my personal listmania.
In recent times, prompted by interesting Web sites such as LibraryThing and the initially mentioned ISFDB, I again started to think about launching a book-related project. This time, however, the main goal was not to simply keep an inventory of books. My vision went more along the lines of creating a database of "links between interesting things". For instance, at some place I had read that one SF author had attended the Milford Workshop - wouldn't it be interesting to know who else was there? Or what about the fact that Alfred Bester wrote his "Demolished Man" with a lot of input from Horace L. Gold? I wanted to record in some way all these relationships between SF authors, stories, places, events, etc., bits and pieces of information that you read about in editorials, articles, introductions to stories and so on, and that you find interesting or intriguing, but that you will inevitably forget two weeks after you have closed and replaced a book on the bookshelf.
So far this vision of an SF knowledge base, of a structured map of relationships in the SF world, has not matured into anything like a concrete project. However, I think it might be a good start to lend a hand to a community effort like ISFDB, to see whether I have the stomach for something larger. Also, a solid bibliographic base like ISFDB can be used by other projects than mine, for citing references. Therefore, if nothing comes of my idea, the effort I am now making for ISFDB will at least not be a complete waste of time.