I use index cards every day to keep my to do lists. I carry them with me to meetings and write what I need to do on one what I need to purchase on another and what I need to follow up with others on yet another. That way as I check off my lists I can just file them with a date or toss the ones that I don't need.
In teaching foreign languages I use many different textbooks and needed flashcards that were lined and multicolored to identify each level of text being thought. The use of multicolor index cards takes out the guesswork of where the information was extrapolated.