So far I think GuruLib has both the best feature (list books borrowed from a library using information pulled directly from the library’s database) and the absolute worst interface. OMG does it suck.
Shelfari OTOH has a really beautiful and, more importantly, functional interface but lacks a good way to mark books that you have read but do not own. Adding the item to your “Reading List” does not count.
LibraryThing’s interface is right about the dead center between the polar opposites of the other two and I cannot find any way to list books that I have only read but do not own.
I could swear there was a fourth site that does similar things to these three, but I can’t seem to remember it. Any pointers to similar sites or comments about these would be greatly appreciated.