Jonathan Lampel, the artist behind the amazing 'Cabin in the Woods' presents a tutorial on realistic grass on Blender Cookie.
This blender tutorial is meant to teach you how to create and add grass to whatever scene you need, by showing you a simple and fast technique.
A few things to note before starting:
This tutorial is going to be using Cycles, but you could take the technique to Blender Internal if you wanted, but you will not be able to follow along exactly while creating the materials. Personally, I just find Cycles easier and faster for a realistic result.
Adding any kind of grass to a scene takes a lot of computer power. I will show you some tricks to save memory and render time, but there is no way around the fact that grass needs to be very dense in order to look good.