Update: This Plugin has been moved to my Projects page and updated to:
PostPost – A WordPress Plugin to place customized content before and after each post in your site or your feed.
I wrote this plugin after trying to add advertising to my feed by myself and getting frustrated. It's my first ‘complete' plugin but I think it's written very cleanly. I really figured out the Plugin architecture utilizing Simple Thoughts' Copyright Plugin. I utilized the construction of Ryan Duff's WP Contact Form plugin to write it much more constructively. It's an excellent plugin to download and learn how to write a plugin since it's so simple.