This tutorial shows how to realistically shatter glass around the point of impact, how to easily set up a rigid body simulation, and how to use render layers for correct shattering.
You need the latest (2.66 RC1 or later) version to use rigid bodies.
Edit: scaling down a cube in object mode works better for the window than an extruded plane.
I felt like I needed to go over two more types of glass to complete your knowledge of glass shattering. If you applied the last tutorial to say, a drinking glass falling on the floor, it would have looked wrong, so here I show you how to shatter drinking glasses and reinforced glass. Sorry if it is a bit redundant, but I hope you learned something new!