Since version 8 of MSN Messenger it's possible to write add-ins with .NET for MSN/Windows Live Messenger. I'm convinced that when the programming models behind this will be mature enough this will open big opportunities for reaching out to all of your Windows Live Messenger contacts, contacts of your contacts, etc.
The ADO Guy Shawn Wildermuth has written an article "Writing Windows Live Messenger Add-ins with .NET" which walks us through the creation of an add-in that replies to incoming messages when the user has the status set to busy. Nice example of an add-in that can be useful when using messenger during business hours.
I think we're still waiting for the one killing add-in scenario. However I'm sure that in the near future we'll see some other useful add-ins become available.
Learn more on developing for messenger on the messenger section of dev.live.com.