Free Flight will change today's air traffic control system by giving pilots increased flexibility to choose and modify their routes in real time, reducing costs and increasing system capacity. This increased flexibility comes at the price of increased complexity. If Free Flight is to become a reality, future air traffic controllers, pilots, and airline managers will require new conflict detection, resolution and visualization decision support tools. This paper describes a testbed system for building and evaluating such tools, including its current capabilities, lessons we learned, and feedback received from expert users. The visualization system provides an overall plan view supplemented with a detailed perspective view, allowing a user to examine highlighted conflicts and select from a list of proposed solutions, as the scenario runs in real time. Future steps needed to improve this system are described.