The Open Library c/o *IA* appeals to me for its huge repository of out-of-print books. PDF-searches for most of them on the Web are futile. Time and again, though, I've seen some kind soul had uploaded the title to IA, saving what might otherwise go to the book boneyard.

Another humanitarian of books is the dev of this ext., without whom there'd be no way for IA users to archive their own copies. In doing so they ensure the viability of the texts. I'm impressed as hell.