Bryce

 Tutorial: Octagon
This is something that is so simple to make yet some people come up with so many difficult ways to do this. Lets begin...
First, create a cube.
Click the chicklet to enter the attributes window. Now click the boolean attribute. Click the to return to the Wireframe.
Press CTL-D to duplicate the cube. Then using the rotate ring, rotate the new cube 45° as seen on the right.
Click the chicklet to enter the attributes window. Now click the boolean attribute. Click the to return to the Wireframe.
Select BOTH cubes and click the chicklet to group them. Once grouped, you should have a group with the clipped corners as indicated by the arrow on the right here. If you don't see these corners, re-check your attributes on BOTH cubes.
Here is our rendered group with the wireframe overlayed on top. The blue cube is our first. The green one is the second.. Using "Intersect" boolean, we only keep the volume where the two cubes overlap.
Our final object rendered. You might wish to save it to your preset library for future use.
Here is a practical use for this object. Here it is combined with some toruses and cylinders to create a nice pillar for the front of a building.