Here on 68 I'd say aluminum is the material we use the most. We mainly use 2X1's .125" in diameter, and 1X1's .0625" in diameter (we do occasionally use .125" 1X1s, but not .0625" 2X1's) for anything structural (chassis and supportive pieces). We'll use .0625" and .125" sheet in addition to various sizes of angled aluminum for less structural things (like the functional/moving parts in our manipulators... such as what pushed frisbees into our shooting wheel this year, and the tube that held them). There are some custom .325" thick plates in a couple places on our robot that we sent out to be water-jetted (mostly on our transmissions). We also used some Diamond Plate on our driver station and robot cart...
I've also noticed the use of lexan sheet in a lot of different places (such as the base for our electrical board, sponsor panels, guides for frisbees to fall into our disc-holding tube, and the outer wall of our shooter). I'd say that is the most commonly used material other than aluminum on my team.
As for your newer question about the use of a CNC mill, we mainly use it to accurately mill out lightening holes in chassis pieces and driver's station (only on 2X1X.125").