The Post-It is soooo outdated, they require a pen, the tremendous effort of writing, and can easily be overlooked by the intended recipient of your message. However, there’s a much better alternative available: a motion-sensing memo.
Record your (short) message, place it in its cradle, and it will play it back when someone enters the room.
Its main drawback is the limited memory, your message can’t be longer than 10 seconds. Leaving that aside, it has some pretty practical uses, besides replacing impractical handwritten notes, you can set a warning that’s played back every time someone approaches. This could help them avoid hidden danger (Warning we are playing Wii Tennis in the next room) or find items you’ve left for them (the pizza is in the oven and the beer is in the fridge).
You can order one for 19.90 Euros at getdigital.de.