Shoutbox

this is the method i've used for a while it isn't fool proof it works a bit

download hopper live
then download convo notifier
then toast them as many time as u can
and if convo notifier responds they are on line if they are not online nothing happens

also
there is a more accurate way of doing the ip ping thing that was mentioned above, of course routers and circurmtances prevent this most of the time atm but it used to work when no one had routers or firewalls.
but u still needed their ip

you use a port scanner and thers a port msn runs on it's like port 8090 or something and if it's alive they are signed in it works better than just pinging the ip address