Office supply maker Avery has a solution looking for a problem: NoteTabs, which are essentially transparent Post-It notes. They're pitching these as a solution for highlighting or annotating materials that will eventually have to be returned in unmarked condition - borrowed books or original documents, for example. I'm sure these needs occasionally exist - what I'm not sure of is whether being able to directly superimpose your notes, rather than slap a Post-It alongside the material, is worth buying a whole separate product. If I'm missing something, fill me in - otherwise, I'll stick (ha!) to my Post-Its.