How do you assign error codes?

Question

How, when developing a medium sized project, do you identify, create and maintain error codes?

I for the life of me can't think of a simple and clean method of doing so. Some of my ideas convert class names and method name into an integer string, but that is way to long to display to the user on top of the fact that method names and class names may change (hopefully not!). Others are just using an incrementing log system (ie. when ever I create a new error message, just add 1 to the last error message id). But that is just completely unorganized.

To be more specific I am talking about error code such as:

Error 401 Unauthorized.