Open-source hardware can play a unique role to spark interdisciplinary research across computer architecture, programming languages, operating systems and computer-aided design. Further, it can enable collaborative engineering among researchers in academic, industrial and government labs. ESP is an open-source research platform for SoC design that combines a scalable tile-based architecture, and a flexible system-level design methodology. With ESP, designers can rapidly prototype a SoC architecture with multiple RISC-V processor cores and dozens of loosely coupled accelerators, all interconnected with a multiplane network-on-chip. Conceived as a heterogeneous system integration platform, ESP can scale to support the realization of massively parallel integrated circuits and chiplet-based systems.