Writing WordPress Plugin

http://codex.wordpress.org/Writing_a_Plugin
http://premium.wpmudev.org/blog/how-to-create-your-own-super-simple-wordpress-plugins/
http://premium.wpmudev.org/blog/how-to-create-your-very-first-wordpress-plugin/
http://wpninjas.com/how-to-create-a-simple-wordpress-plugin/