I just loaned this out, and I guess I could make a note about who I lent it to here, but that seems like an odd use for the public discussion space for this book.
Hear ya loud and clear! I've been feeling the same dilemma with things that I technically own but don't physically have with me these days (while my stuff is packed up in SF).
Delicious Library seems really heavy, and I'm usually not near my computer when I'm loaning something, so it's easier to just tap it into my phone. Ideally I would have something that already knows what I have and who my friends are, and that's available on mobile and desktop. A number of the existing lending sites require that you lend through them somehow, or you can only lend to people who have friended you on the site… I don't want any of that necessarily, just something lightweight to keep track of loans I've already made outside the system.