A Makerbot Giveaway entry.
This is a stereoscope used for viewing stereo pairs (two images that together make a 3D image) arranged parallel style (not cross-eye). Thanks to its central pivot, it can be used with a wide variety of image sizes. It's ideal for viewing pairs made of two 5x7" photo prints, or images approximately that size on a monitor. It's also fun to just plain look through the viewer, because it increases your depth perception by effectively moving your eyes farther apart.
But that's only half the fun. I've included a collection of stereo images I made especially for this viewer (see "Kongoscope_3D_Image_Funpack.pdf").
They're good viewing on your monitor or printed on 8.5 x 11" paper.
But that's not all! I wrote a light-hearted user manual (see "Kongoscope_USER_MANUAL.pdf") that should make assembly easy and fun. It also has viewing tips. While writing the manual, I made up a fantasy company, KONGO!, that's marketing this device. See their comic book advertising over there on the left.
I hope somebody has fun with this!
EDIT OCT. 6, 2010: By request, I've added the original Blender file. Note that there are reference objects in the file not meant to be printed, like mirrors and screws.