Live Notifier has been mentionned and linked to a lot. There are many threads about it. And many threads explaining the notifications and their accuracy.
As for combining something like this to have a who is appearing offline tool, that is exactly what this whole thread is about and which is explained in detail already in this very same thread.
EDIT (because I wasn't very clear I think, sorry for that): The use of open window notifiers, like Live Notifier was already discussed indirectly in this thread. And Steel Froggy already mentionned the use of open convo notifiers, like Live Notifier, explicitly to make such a "appear offline notifier". Although possible, they still would be somewhat inaccurate (but not so inaccurate as open window notifiers) as already explained in this thread.
For more info about open window notifiers, see: "CookieRevised's reply to Question regarding a "close msg window logger"".
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