basil - It is very important to cook with fresh basil. Don't use the old withered basil that's lying around in your fridge, or you'll get a brown pesto.
Variations - You can use the above recipe and replace the basil or walnuts with one of the following ingredients:
-Arugula, spinach shoots, parsley, coriander, carrot tops, beet tops.
-Pine nuts, cashew nuts, almonds, hazelnuts