We have developed a novel and generic open-source platform - Apollo - which simplifies the design of custom Advanced Telecommunications Computing Architecture (ATCA) blades by factoring the design into generic infrastructure and application-specific parts. The Apollo "Service Module" provides the required ATCA Intelligent Platform Management Controller, power entry and conditioning, a powerful system-on-module computer, and flexible clock and communications infrastructure. The Apollo "Command Module" is customized for each application and typically includes two large field-programmable gate arrays, several hundred optical fiber interfaces operating at speeds up to 28 Gbps, memories, and other supporting infrastructure. The command and service module boards can be operated together or independently on the bench without need for an ATCA shelf.