Invisible Emissive Material

In the Cycles renderer, let's assume that you want to light your scene with an emissive mesh, however, that mesh is in the way of how you want to view the scene.  Hiding the mesh will also remove the light effect, so we need to make the object invisible to the camera, but still light the scene.

With the emissive object selected, in the Object panel, under Ray Visibility, uncheck the Camera box.

Now the scene is still lit by the mesh, but the object is invisible to the camera.