Wikipedia
Actor-Lab is a program that visualizes the Lego Mindstorms internal logic through visual flowcharts. The ideas underlying actor-lab are derived from the event-driven, message-passing computer languages that began with the actor based languages devised by Hewit. Actor-lab requires constant communication with the RCX brick via the infra-red link. This means that you have to mount the RCX vertically for models that move about, and hold the tower in some form of gantry crane. Actor-lab shows you in real-time what is happening on the RCX, including simulation of motors, sensors, lights, and sounds. You can also add controls at the bottom of the screen to give you real-time control of your program/model. There is a similar program from Lego called Robolab. In Robolab, you can actually download programs to the Lego RCX.