(I'm still using Delicious Library also, since I think it's neat to scan ISBN bar codes with my iSight cam, plus it has some extra features like checking out books. However it's a Mac-only program.)
Edit: It should go without saying that I have read many more books than I currently own. For example I read all of Pohl's Gateway series and a good number of McCaffrey's Pern adventures (for better or worse) while living with my ex (a huge speculative-fiction buff), but don't have anything to show for it, less a not-so-good recent Gateway novel which is tagged for garage sale. I don't even have my own copy of one of my favorite books ever, Le Guin's Wizard of Earthsea, because I'm holding out for a decent hardcover edition.