This is one of my favorite resources for writing a WordPress plugin. In his post, Francis Yaconiello, provides a great overview of how to setup a plugin in a logical fashion without over complicating things.
I’ve noticed that a bunch of the how-to-write-a-plugin articles out there focus on demonstrating the minimum amount of code needed to get a plugin going. Not many focus on good plugin structure or convention. This tutorial explains how to create a class based WordPress plugin that makes sense.