This inkscape extension installs a new menu entry under "Extensions" -> "Modify Path" -> "Flat Projection..."
Apply a 3D rotation matrix to 2D objects, then apply an axiometric projection back into the 2D plane. This implements parallel projection. The side walls use algorithm T as described by Donald Knuth.
(For perspective projection see Tav's perspective extension.)
To compose arbitrary scenes, objects can be rotated before perspective is applied: