IMWatching lets you enter the screen-names of up to 40 people whose activity on AOL Instant Messenger you want to track. The site then collects what it calls “presence data,” or information on when those users sign on, sign off, go idle or flag themselves as being away. Usually, IM software only collects the presence data while you are logged on, and it forgets whatever it knows as soon as you quit. IM Watching works 24/7!

The sites claims that it can help you know:
…how long ago your spouse left work for home?
…how much time you (or your kids) spent online this month?
…whether a coworker is about to walk in the door or is home asleep after a grueling all-nighter?
…what time your friend is likely to return from lunch?

Besides, the users tracked are not notified!

From The New York Times.