Unable to remove the base using the OpenScad version of the 'hollow cube' (see ancestor) I went with what I knew and drew it out again in Sketchup. This is a 30mm cube with all hollow faces and 4mm square bars. As orionwnix said previously, it's great for many 3d printer tests including ooze/retraction, dimensional accuracy and squareness, and of course bridging!
Short video of this cube being printed on my blog, specifically the challenging bridging stage: http://numbersixreprap.blogspot.ie/2012/09/bridging-gaps-video-clip-included.html
Download the .stl, slice and print. (that gives a 30mm/4mm hollow faced cube).
If you want to scale or modify in any way I've also shared the Sketchup file.