is there a built-in filter or extension in Inkscape to create a similar fold/3D effect? I mean the appearance simulating a foldout leaflet/brochure.
Thanks for any hints!
It looks pretty trivial to draw by hand. Here's a few tricks you could use to make it a bit easier:
Start by setting up a grid for your document (via File → Document Properties... → "Grids" tab) and turn on "Snap to Grid". That way, you won't have to take so much care in getting things lined up correctly.
Also (or alternatively), you could set up some vertical guides to mark the folds, and some horizontal ones where you want the midline and the top and bottom of the brown/blue areas to go.
Once you've drawn the brown area, copy and flip it to create the blue one (or vice versa).
To create the shading in the folds, draw a tall black rectangle, set its opacity to something like 10% and position it over the fold. Then convert it to a path and move the bottom corners to match the corners of the blue area. Assuming you set up a grid and/or some guides for those corners earlier, this should be quick and easy. (Alternatively, you could use the path intersection tool to do it, or even set an explicit clip shape, but that seems like more work in this case.) Copy it and adjust the corners for the other folds.
Here's a set of screenshots showing how to do it (click for larger version):