otoy comes the OctaneRender add-in for Revit...
"OctaneRender is an extremely fast physically based photorealistic renderer that uses the GPU to create stunningly lifelike images of your scene in near real-time. The OctaneRender for [Revit] plug-in allows real-time interaction with the scene, while at the same time rendering at final quality: you can change the camera, lights, materials, textures and the position of objects in your scene and see the final quality result in near real-time. Because of its incredible speed, quality and ease of use, OctaneRender is an excellent tool for architectural visualization. OctaneRender requires an Nvidia GPU to run. This app is a demo version of the OctaneRender plugin, which is fully functional and has no time limit, but the rendering contains a watermark and the maximum render resolution is limited to 1000x600."
otoy's website claims rendering speeds "up to 50x faster than CPU based unbiased renderers."
There's more information available on the Autodesk Exchange Apps website.