funny you should ask. I put many scratches in the windshield when scrubbing it with a scotchbrite pad that had a piece of rust in it. What you want is cerium oxide. It can be gotten through JCwhitney in a kit, with the powder and the polish wheel that goes on the end of a low speed drill, for $30. It's available through other channels but for more $$ though. I will be using it this weekend to fix my screwup, I'll let you know how it goes.
Glass polishes like zaino (which i have) or other off the shelf stuff don't work at taking scratches out. The glass is too hard and the polish too soft to cut, especially when using by hand.
The only catch with cerium oxide is the scratch cannot be deep. If you can feel the scratch with your fingernail at all, then it's too deep. You can only polish out fine scratches that can be seen but not felt, otherwise you will distort the glass by polishing (cutting) into it too much. And the other thing is to polish at a slow speed so you don't build up heat and burn the glass.