Emesene messenger
Emesene is a platform independent MSN Messenger client written in Python+GTK and distributed under the GNU GPLv2 license. The main idea is to make a client similar to the official client but with a simpler GUI and a nice look.

OS Independent (Written in an interpreted language) / Currently in Beta stage(testing for final vesion 1.0).

Windows Installer & Portable Download

Wiki & other downloads

Emesene is hosted @ SourceForge

Visit their forums for patches, themes, plugins, etc.