A little box, on the game screen where you can leave notes to yourself. Could be floating (like glovechat) or static.
Notes to self may be good but if there is any chatting involved, the notes are usually buried quite fast. And cant be reviewed easily once eliminated.
Personal notes in team games cannot yet be done.
I see so many uses for such an addon.(use your imagination)
It would be like "note to self" on steroids!