Integrated our e-commerce site with BlueHornet's (www.bluehornet.com) Email Marketing service.  This is an on-going project and has many components.  I'll outline a few of the big pieces.

1. Manage My Preferences - This is a UI that I built that allows the customer to manage their communications preferences.  It also synchronizes any changes between the BlueHornet system and our e-commerce system.  It utilizes the BlueHornet API to communicate data.
2. Signup Forms - All the customer signup forms in our e-commerce system where converted to submit their data straight to BlueHornet.  This was done for a variety of reasons.  It delegates processes and data related to signups to the Blue Hornet servers.  It consolidates data and it also provides instant access to the data.  I built a custom admin section for managing the forms and modifying what data is sent along with them.  This way we can segment users based on where and how they signed up.
3. Email Conversions - Customers who click on items in marketing emails are tracked all the way through the order process and that conversion data is sent back to BlueHornet so that the e-commerce team can see how effect their marketing efforts are.  I implemented the process as put forth by BlueHornet and batched the processing of conversion data to be run via the cron schedule.
4. Order Segments - Customers are segmented by their order history.  Products and categories are given segment ids and when an order is registered in the system any products or categories that contain those products are converted into segment ids and that data is then sent to BlueHornet.  This allows the email marketing team to send targeted email marketing pieces to users who have bought certain items or expressed interest in certain segments.