Orthographic Frustum
Perspective Frustum
Orthographic Scene
Perspective Scene
You can run this interactively using Geomview. Type "geomview frustdemo" to start up. There are two windows, the main one showing the scene (purple background) and the one showing the frustum (green background).
Type "vp" in the window with the purple background to toggle between perspective and orthographic mode. In orthographic mode you'll need to reset the field of view to something small, looks like a value of .7 will roughly match the picture. (Enter .7 into FOV text field of the Cameras panel, or type "0.7vv" in the window with the purple background.) You can move the camera back and forth by entering fly mode (hit "f", drag with the middle mouse to forward/backwards, drag left to turn your head). Note that there's a lot of geometry in the scene, so it will be slow on a low-end workstation (one frame a second on an SGI Indy).
I originally made this for the expository talk "Visualizing the Invisible" at the Weisman Museum of Art in 1994, the same year I learned to spell it "frustum" not "frustrum"...