If you're interested in writing apps that use the MSN Messenger protocol, Chris Sells has written some simple C# code to send messages.

Also, take a look at MSN Messenger Protocol(Practical Implementation) a.k.a How to make your own MSN Messenger In VB.

I've started my own asynchronous event-driven code library in C# which is progressing well, but not quite ready for publishing yet. Check again soon...