This paper introduces a new framework for building digital library collections and contrasts it with existing systems. It describes a significant new step in the development of a widely-used open-source digital library system, Greenstone, which has evolved over many years. It is supported by a fresh implementation, which forced us to rethink the entire design rather than making incremental improvements. The redesign capitalizes on the best ideas from the existing system, which have been refined and developed to open new avenues through which digital librarians can tailor their collections. We demonstrate its flexibility by showing how digital library collections can be extended and altered to satisfy new requirements.