Recent work has shown the potential for lights to act as simple, yet expressive signaling mechanisms for use in a variety of human-robot applications. However, the wide range of robot shapes and sizes makes it difficult for researchers to quickly prototype and evaluate different light configurations and signal designs. In this work, we present the design of ModLight, a modular research tool consisting of a set of low cost light blocks that can be easily reconfigured to fit a myriad of robots and applications. ModLight also provides researchers, designers, and students with open-source software tools that enable them to visually design new signals and easily integrate them into existing systems. This work also serves to motivate the need for further research in developing light behaviors for use in human-robot interaction. Towards this goal, we present our design rationale including a brief analysis of the signaling needs of several robots and applications.