Recently, Marilyn K took the time to develop her first WordPress plugin. This took skill with MySQL, PHP, and the WordPress API. She gives this plugin away for free from one of her own websites.
Some example article titles include:
WordPress vs. Joomla vs. Drupal
Seven WordPress Plugins Your Local Business Site Needs
How to Turn WordPress into a Directory with Free Plugins
Content Manage Systems or Code Your Own
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Five Improvements You Need to Make to Your Business Website