Nokia has added Googletalk, Skype, Ovi chat and Jabber onto the n900 contact list. Msn seems to be missing, but there are plug-ins which you can easily install and add msn into your contacts menu. You can see whos online and chat with them
But, if your looking for a more windows live interface, aMSN is available. It allows offline messaging, voice clips and webcam support. Thats right, you can webcam using the front camera with your loved ones.
Here are some screenshots
You can download aMSN here
and the MSN messenger client for N900 here