I believe they manufacture the glass without Iron in it which makes it very clear. But it also slightly weakens the glass from my understanding. It costs a little more and might be worth the extra bucks if your building a high end system. The Manufacturers usually just use the iron free glass on the front and side panels.
Starphire is a brand of low-iron glass. Mos people who see it in person say it is even more clear than standard low-iron glass. I'm ordering my tank next week and the front viewing pane will be Starphire because the clarity truely is amazing.