GNOME Human Interface Guidelines (HIG)
GNOME is currently documenting the design patterns used in GNOME 3 applications on the HIG wiki. This is very much work in progress. An updated Human Interface Guidelines document focusing on GNOME 3 will be released in due course.(ref)
Supporting the user experience in free/libre/open source software development (PDF)
PhD thesis by Paula M. Bach (2009, Pennsylvania State University). 209 pages.
Usability in Free Software
A guide by Jan-Christoph Borchardt. From the abstract: "For a software to truly be free, people need to be able to easily use it without help. This is a quick start to usability testing in a distributed and independent development environment: A collection of usability methods applicable especially in free & open source software projects. No matter if you are a developer, usability person or just enthusiastic about making software easier to use, this document should help you get going."
How to not report your UX bug
Presentation (45 min) by Fabiana Simoes at GUADEC 2013 (GNOME Users’ And Developers’ European Conference, held annually in cities around Europe). From the abstract: "Empowering users to proactively contribute to FOSS projects by reporting UX issues is not only a way to leverage UX in FOSS projects, but one that fits the FOSS community values. By referring to the users' personal experiences, designers in FOSS communities may build and inform their design decisions in an open and transparent manner. Not only that, but this is also a way by which we can engage our users in the values embedded in our community. For this to be possible, however, the information provided by users in their reports and the information designers need have to be aligned. DOs and DON'Ts for reporting UX bugs that are actually useful for designers and feasible for them to act upon."