aMSN have developed the MSN messeger which can be installed on the new Nokia N900. This application is a for free. I don�t know whether the MSN messenger works on the Nokia N900 which is available on OVI. If it is not working, then have a try.

Some screenshots for you.

aMSN Features:-
    * Offline Messaging
    * Voice Clips
    * Display pictures
    * Custom emoticons
    * Multi-language support (around 40 languages currently supported)
    * Webcam support
    * Sign in to more than one account at once
    * Full-speed File transfers
    * Group support
    * Normal, and animated emoticons with sounds
    * Chat logs
    * Timestamping
    * Event alarms
    * Conferencing support
    * Tabbed chat windows