Am I the only one (surely not) who collects bookmarks (and actually uses them all)? Not concurrently, of course (I don't read that many books at one time!). I made a little box, covered with arty wrapping paper, in which I stand all my bookmarks (there are probably about 60 by now). As I finish a book, I put the bookmark I've used at the back of the box, taking the one at the front for my next book. They all make their way back round eventually.
In answer to your question, Calendargirl, I use all kinds of markers - library opening times, bookshop freebies, picture cards from art galleries, local festival advertising cards, etc. If I receive a particularly nice greetings card, I'll chop the front of it down to an oblong. I can't bear to be without a bookmark (just in case) so I carry a couple in my handbag....