It can probably also be put to less nefarious uses. Here's one that comes to mind for a web based chat client: You can use it to detect if a user is currently

It can probably also be put to less nefarious uses. Here's one that comes to mind for a web based chat client: You can use it to detect if a user is currently at his/her desktop or away. Unless the chat window is the current active window, I think this would be hard/impossible to do otherwise.