This paper examines the emergence and evolution of user innovations in Open Source Software (OSS) development, with focus on usability innovations. Existing literature on user innovation and usability is reviewed, after which usability innovation is empirically explored in OSS development. The interpretive case study shows that usability innovations emerge and evolve in OSS development. They emerge after a user recognizes a need, after which she invents a fix to meet the need, thereafter needing a developer to realize the fix in the OSS. Afterwards, the user experiments with the solution and may provide feedback, which again may lead to the developer adjusting the OSS accordingly. The process is characterized as a collaborative negotiation process among the users and developers. The results also reveal that the usability innovations may be need, opportunity or creativity based, and connected to improving efficiency, effectiveness or satisfaction. Implications both for theory and practice are discussed.