Landell is a Instant Messenger for the GNOME Desktop. It is written using C# and Telepathy (though the use of Tapioca#, which is a high-level convenience library based on Telepathy#).
Screenshots are available here: http://tapioca-voip.sourceforge.net/wiki/index.php/Screenshots
And the project website is: http://tapioca-voip.sourceforge.net/wiki/index.php/Landell