Within the "Writing a Plugin" section of the codex (http://codex.wordpress.org/Writing_a_Plugin), the following statement is made:
To learn the basics about how WordPress Plugins are written, view the source code for well-written Plugins, such as Hello Dolly distributed with WordPress.
I totally agree with the advice to learn from other well-written plugins, however, the Hello Dolly plugin is pretty basic. That being said, what are some other examples of well-written plugins that are more complex? What are the plugin design patterns that have emerged that developers should be following? What are some best practices for plugin code organization?
I'd love to hear some suggestions of other well-written plugins that I can model my plugins after.