With a few free Unity assets (and optionally one paid asset) you can turn your Unity scene view to look like Google Sketchup, with a flat, cell-shaded look that displays a thick outline around meshes. I’ve found that this gives a clearer overview of the scene while editing, especially being able to easily distinct the borders of a mesh is useful and it helps detecting wrongly flipped normals more easily.
All that’s needed is either the Toon shader from the Unity Standard Assets or optimally the Edge Detection camera effect from the Standard Assets used with this nifty asset named Scene FX View (which allows you to see your scene in the editor as it would be appearing in the game). The second option looks arguably better since the outlined Toon shader still has gaps appearing around the corners (it looks more like a very close 3D drop shadow than a real outline).
All you then have to do is create a scene, give it a complete white skybox as background, add a camera to it, add the Edge Detection script to the camera and turn on the Scene FX View. A nice prototype texture helps, too.
Here’s a free asset package for download that includes this all (sans the paid Scene FX View of course). It also includes my prototyping materials and textures …