Extending Contact Form 7
Contact Form 7 is a wordpress plugin that allows users to easily put forms into wordpress pages. These forms are very basic, but very extensible. I recently had a task to change Contact Form 7 to be a 2-step form. In doing this I found a lot of hooks and help. In the end created my first official wordpress plugin.
Posts explaining how to send a form to another form: http://wordpress.org/support/topic/plugin-contact-form-7-use-cf7-with-action-another-wp-page
Guide to customization: http://hecode.com/easy-contact-form-7-guide-and-customization/
Add a hidden field: http://wordpress.org/extend/plugins/contact-form-7-modules/