Peter Wooster

This plugin allows editors to request a promo code and optional email address from users to get access to a document and track its usage.
Features for Editors
<ul>
<li>a short code [promo] is provided that will display a form that requests a 
promo code and optionally an email address from your users and then takes them to a url.</li>
<li>a summary of the accesses using this code is displayed to editors by the short code</li>
<li>editors can clear the logging from the short code</li>
<li>there is no back end admin page </li>
</ul>
Short Code Arguments are:
<ul>
<li>code= a string that must be entered access the document, required</li>
<li>document= a url to the document that you want to track, required</li>
<li>name= the name to display for the document, required</li>
<li>email= ‘yes’ to request the user’s email</li>
<li>id= the id on the form tag, useful for CSS, default is ‘promo’</li>
<li>promolabel= the label for the promo code input field, default is ‘Please enter promo code for name’</li>
<li>emaillabel= the label for the email input field, default is ‘Please enter your email address’</li>
<li>liststyle= the style or class for the ul, default is ‘style=”list-style:none”‘</li>
</ul>

Simple Promo Code

A very simple plugin that allows an editor to track accesses to a document by promo code.

wakamin

Turn your WordPress site into coupon central.
<h3>How to Use?</h3>
<ol>
<li>Add stores from Coupons menu then click the Stores menu link.</li>
<li>Add categories from Coupons menu then click the Categories link.</li>
<li>Add new coupon from Coupons menu then click Add New menu link.</li>
</ol>

Coupon Central

Turn your Wordpress site into coupon central.

Simple Promo Code vs Coupon Central