http://www.videogamesartwork.com/games/halo-reach

Found this neat little site while I was googling around. I know most of these are from the art books and whatnot but if you never got a chance to pick up the artbooks this is a good way to check out the concept arts for these games too. also gives links to buy them too!

For Halo 4, its neat to see the flood armor concepts and I learned something new about how the Diadict "died". and as for Halo Reach, I finally saw what Rosenda's armor looks like and it looks like they had somewhat of an idea of what Noble 6 was supposed to look like underneath the helmet.