I really like Oliclips. The magnets are pretty strong and so you can use them as bookmarks, money clips, paperclips, fridge magnets, pen loops or to close food packets. I have even used them as hairbands in an emergency.
I prefer the big ones for my A4 teaching papers and the small ones for my notebooks and calendars. My oldest ones have been in daily use for more than two years and are still going strong!
There is a constantly changing selection, with some beautiful limited editions. At the moment, these are available for order and pre-order: