I've never heard of an open-source solution that has gone through formal usability tests.
WordPress has.
http://wordpress.org/development/2008/10/usability-testing-r...
WordPress has.
http://wordpress.org/development/2008/10/usability-testing-r...
http://live.gnome.org/UsabilityProject
KDE too, apparently: http://usability.kde.org/