Shoutbox

You could try convo notifier or live notifier... (someone post a link? gtg quickly)
It show when someone opens a connection to your account, but you won't see it if they do not connect, so you can't just see if someone is appearing offline
It won't show when you're blocked by someone, becaus then that contact won't make a connection to your account

Next time try to search better
there are dozens of this posts