Ubuntu has lots of multi platform chat client but if you are looking for msn alternative, emesene is absolutely best one. Installation is easy;

sudo apt-get install emesene

If you are using light panel with dark icons like me, you are searching for mono theme for it it is also easy to replace.

–> Download; http://www.box.net/shared/rmenzd857l

–> Move the folder to the usr/share/emesene/themes (after writing to the terminal gksudo nautilus)

–> Choose emesenemonochrome theme from the preferences/themes of the emesene.

That is All !