Information technology has entered the age of heterogeneous computing. Across a variety of application domains, computer systems rely on highly heterogeneous architectures that combine multiple general-purpose processors with many specialized hardware accelerators. The complexity of these systems, however, threatens to widen the gap between the capabilities provided by semiconductor technologies and the productivity of computer engineers. Open-source hardware is a promising avenue to address this challenge by enabling design reuse and collaboration. ESP is an open-source research platform for system-on-chip design that combines a scalable tile-based architecture and a flexible system-level design methodology. Conceived as a heterogeneous system integration platform, ESP is intrinsically suited to foster collaborative engineering across the open-source hardware community.