The Durham Wildlife Trust required a sign up system that would allow new members to sign up and redeem any promotional code that was on offer at the time. The system was fully built by hand and incorporates client side validation as well as paypal integration.
I was charged with the build of the site and the membership sign up. The site is based on WordPress and the sign up was built as a separate entity. I chose this path as we wanted to keep all of the sign up data separate and have no interaction with the WordPress database.
If you want to take a look at the project you can visit The Durham Wildlife Trust.