OpenRelativity is an open-source toolkit to simulate effects of special relativity by varying the speed of light. Developed by the MIT Game Lab, it contains open-source code for public use with the free and paid versions of the Unity engine. The toolkit was developed during the creation of the game A Slower Speed of Light.
See OpenRelativity in action in the following game:
A Slower Speed of Light is a first-person game in which players navigate a 3D space while picking up orbs that reduce the speed of light in increments. Visual effects of special relativity gradually become apparent to the player, increasing the challenge of gameplay. Players can choose to share their mastery and experience of the game through Twitter. A Slower Speed of Light combines accessible gameplay and a fantasy setting with theoretical and computational physics research to deliver an engaging and pedagogically rich experience.