In this paper we present a robust software infrastructure for metacomputing. The system is intended to be used by others as a building block for large and powerful computational grids. Much effort has been taken to develop a fault tolerant architecture that does not exhibit a single point of failure. Furthermore, we designed the system to be modular, lean, and portable. It is available as open source and has been successfully compiled on POSIX and MS Windows compliant platforms. The system does not origin from a laboratory environment but has proven its robustness within two large metacomputing installations. It embodies a modular concept which allows easy integration of new or modified components. Hence, it is not necessary to buy into the system as whole. We rather encourage others to use only those components that fit into their specific environments.