I think I'm obsessed with bookmarks - I have about 70 different ones just now, plus a stack of plain chipboard ones that I'm going to customise myself. I have an awful habit though, of not using them and instead using old train tickets, receipts or anything else that happens to be lying nearby
If I lend out any of my books I always make sure to put a bookmark in and give the standard lecture about not folding down corners - my pet hate