Here are two of my 3D quilts, "Impossible Triangle" and "3D Color Study".
Careful use of color values and placement gives a three-dimensional look and feel to these quilts.
They look difficult (if not impossible!) to make, yet both are done with all straight line seams. The top one is made out of equilateral triangles sewn in slanted columns. The bottom one is made the same way, with the addition of the thin strips at the "edges" of the boxes.