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