One of my clients recently requested adding any applied coupon codes or gift certificate codes to the invoice printed from Magento.  This is actually a pretty quick and easy change.  Below are the steps required to implement this change in Magento 1.4.1.1.

1. Copy /app/code/core/Mage/Sales/Model/Order/Pdf/Invoice.php to /app/code/local/Mage/Sales/Model/Order/Pdf/Invoice.php
2. Find the line $page = $this->insertTotals($page, $invoice); in the getPdf function.  This was line 107 for me.
3. Add the following:

   /* added by Wright Creative Labs */
   /* print coupon code on invoice */
   if($order->getCouponCode()!=""){
       $this->y -=12;
       $page->drawText('Coupon Used: '.$order->getCouponCode(), 450, $this->y, 'UTF-8');    
   }
   /* print gift certificate code on invoice */
   if($order->getGiftcertCode()!=""){
       $this->y -=12;
       $page->drawText('Coupon Used: '.$order->getGiftcertCode(), 450, $this->y, 'UTF-8');    
   }

4. Save.
5. That's it!  Go to the backend to test it out... Sales > Invoices and then try to print an invoice with a coupon code or gift certificate.