On the bookshelf I used colored-popsicles and wrote on them with permanent marker. This way all the subjects remain separate and easily accessible. I have all math related books, workbooks, activities and puzzles in one shelf and so on.
I use this "paper tak" to stick things on the walls and things. I like them more then tapes or double sided tapes because it doesn't effect the walls/leave marks on them. They even stick Bristol Boards!!
What are some ways you label your items at home for your children and adults?