I like noyb's as well, but I'd like to see a bigger version of it (with one of the transparent materials) and a tiled floor.
Maluminas: I'd also be interested to see your model with a material that shows whatever's inside (just a sphere, it looks like, so maybe it's not a big deal). The variety of surfaces and reflections is cool, but the number of lights in the current setup makes for a pretty busy image.
ThatDude: I like the idea, but the sphere that actually serves as the material preview doesn't feature very prominently in the image as a whole - like well under a quarter of the total image. Could you maybe shrink the "stand" a little bit (or even a lot) and make the sphere bigger (and add the indigo logo

)? Also: something on the floor? I don't know. Just my two cents.
Everyone: one other thing to consider from the Maxwell material preview: the "Sub Surface Bar" is a nice way to have some sort of precise way of seeing how the underlying surface is showing through - this is only marginally interesting with smooth clear/colored glass materials, but it will become more relevant with bump mapping and other things that affect the surface texture of a transparent material. See fmell's
ground glass material preview for a great example, or jomaga's
wine bottle. Without some variation of detail on the underlying surface, there's really no way to see exactly what's going on.
Great job people. Keep up the good work.