From the About page:
The idea of OpenUsability originated at the KDE developers conference in 2003. Jan Muehlig and his Berlin-based company relevantive had performed a comparative usability study of KDE 3.1 and Windows XP. The developers appreciated the work of the usability professionals, and a discussion came up how to involve usability feedback continuously with development. So far, it had been difficult for both sides to get in touch with each other: On the one side, self-acclaimed usability experts had reduced the developers' trust in the effects of usability, and on the other side trustful usability specialists often had problems to get in touch with the right people in a project.(Note: the comparative study between KDE and Windows XP no longer exists on the relevantive.de site, so I have dropped the links.)
OpenUsability came to fill this gap. Since 2004, Open Source Software Projects can register to get usability support. A growing number of usability practitioners started to work under the label "OpenUsability", and provided the projects with usability feedback, attended developer conferences and fairs.
They provide a list of projects that are specifically targeted to help certain open source programs: ArgoUML, Edyk, GCstar, ProjectPier, and so on. However, I don't see any general guidelines for how open source developers can apply usability practices into their own work, or successful ideas for good usability.