about MSN messenger:
Java MSN real status viewer (I can't get it work)
Triople can detect if a msn user is online of not. (web interface)
and you can even send message to an user who already blocked you by changing your nick name to a magic string.
(a special software to change nick name is required)

this "Online Status Indicator server" can let users know if someone is online or not.
but can't detect real status.