This paper explores the relationship between paid labour and users within the Zooniverse, a crowdsourced citizen science platform. The user activities involve the collective categorisation of large datasets, mainly relating to images that cannot currently be analysed algorithmically. However, unlike other examples of micro-tasking, there is also the possibility for individual users to make serendipitous discoveries. It was initially established by a small group of academics for a single astronomy project, but has now grown into a multi-project platform that has engaged over 1.3 million users so far. The growth has introduced different dynamics to the platform as it has incorporated a greater number of scientists, developers, links with organisations, and funding arrangements. The different organisations and funding requirements each bring additional pressures and complications. The scientists come from a research-led university context, while the developers are drawn from more of a start-up culture with an emphasis on open-source ideals. The relationships between paid/professional and unpaid/citizen labour have become increasingly complicated with the rapid expansion of the Zooniverse. The paper draws on empirical data from an ongoing research project that has access to both users and paid professionals on the platform. This combination of ethnography, in-depth interviews, and quantitative data combines to provide new insights into the organisation and processes of this large citizen science platform. The Zooniverse case study provides an important starting point for understanding the dynamics of paid and unpaid work in the context of peer production. There is the potential through growing peer-to-peer capacity that the boundaries between professional and citizen scientists can become significantly blurred. Crowdsourcing can allow the complex tasks involved in data analysis to be collectively achieved, yet there remain limits to the contribution that individuals in the crowd can make. The findings of the paper therefore address important questions about the production of value, ownerships, and the politics of open source acts. These are considered specifically from the viewpoint of the users and therefore form a new contribution to the theoretical understanding of crowdsourcing in practice.