This paper describes StarT-Voyager, a machine designed as an experimental platform for research in cluster system communication. The heart of StarT-Voyager is a network interface unit (NIU) that connects the memory bus of a PowerPC-based SMP to the MIT Arctic network. The NIU is highly flexible, with its set of functions easily modified by firmware or by programmable hardware, making it possible to compare different communication interfaces and implementation strategies on a common platform. Its flexibility comes from a fast embedded processor and large, fast FPGAs that surround a high-speed protected communication core. Its efficiency comes from a set of primitive operations that are implemented in hardware and are designed to reduce the firmware overhead. Our initial configuration of StarT-Voyager implements four forms of message passing along with S-COMA and NUMA shared memory support. With experimentation on the machine, it can be reconfigured to introduce new mechanisms improving usability and performance.