http://codex.wordpress.org/Writing_a_Plugin
Essential WordPress Plugin Development Resources, Tutorials and Guides
20 Tutorials to Develop WordPress Plugin
10 Tutorials To Kickstart Your WordPress Plugins Development
Create a Custom WordPress Plugin From Scratch
http://www.problogdesign.com/wordpress/a-novice-guide-to-wordpress-plugin-development/
http://www.slideshare.net/chzigkol/wordpress-plugin-development-short-tutorial-presentation
http://www.devlounge.net/extras/how-to-write-a-wordpress-plugin