Happy almost Halloween! I was just completing the write-up of one of my SketchUp projects of the month, which involves using the Intersect Faces tool to cut a window into a rocket.
You can apply this technique to almost anything. So if I want to make a jack-o-lantern, I can download a pumpkin from the 3D Warehouse (there are many), and create some prismatic objects to represent the eyes, nose, and mouth. It's easier to create these objects in blank space then move them into place. And everything should be exploded first, if needed.
Select everything, right-click on any selected face, and choose Intersect Faces / With Model. This gives you the intersection edges you need to make the cutouts. Then you can trim away all the extra edges and faces. (My cutouts go all the way through, which you might not do with a hand-held knife on a real pumpkin!)
There are already some great Halloween pumpkin models in the 3D Warehouse. These are awesome:
Now, I'd really like to see someone model this one:
Anyone can design anything in 3D! http://www.3dvinci.net/