These are essentially note sized post-able whiteboards or simply reusable post-it notes. At my workplace we seem to use a ridiculous amount of post-it notes. Although must are discarded in the paper recycling, perhaps there is a greener solution.
I started with keeping a small DIY whiteboard (there are several here
at Instructables to try) by my desk and asked my co-workers to use that if they needed to leave me a message. I've had some success with that though I thought it would be great if I could use something similar to leave messages on other people's desks, documents, monitors, doors, etc. So I made these laminated notes with Blu-Tack stuck on the back.