A replica of the 25mm construction rails sold by ThorLabs (http://www.thorlabs.de/NewGroupPage9.cfm?ObjectGroup_ID=194). The channels along the faces of the rail will accept M6 nuts in order to build larger structures, and several rails can be connected together using some rods to form larger rails.
This model was created (not merely taken from the ThorLabs website, where it is available as a .step file) in SketchUp using dimensions provided on the ThorLabs website along with some guesses at the underlying construction geometry.
If you have ever had the pleasure of using ThorLabs construction rails before, you will know how sturdy and useful they are. The version I printed is very strong - much stronger than I anticipated - and I imagine it would print well with 2 or 3 perimeters and no infill. I think the model I printed was using 2 perimeters and a 40% infill - looking back, this was probably overkill and a hollow model with 3 perimeters would likely have the same strength, print in around the same time and be a little cleaner.
This model is 9Â cm tall (in order to be printable on a Printrbot Jr), but can easily be extended in the SketchUp model using the push/pull tool on the top face. Larger rail lengths could easily be made by printing several of these rails and joining them with 4x 4Â mm diameter rods in the corner holes.