The Product Portals leverage a single service for retrieving interesting/relevant coupons. These coupons can also be queried via an API.

The default call (visible in the browser) will return 10 random coupons:
http://coupons.wishpot.com/

To get coupons for a given website, search with it's domain name like this:
http://coupons.wishpot.com/?merchant=drugstore.com

To get coupons by a given keyword, search like this:
http://coupons.wishpot.com/?keyword=flowers

Coupons are in the following XML format:

<coupon>
    <alt-category>None</alt-category>
    <alt-title nil="true"/>
    <category>Cookies and Cakes</category>
    <coupon-code nil="true"/>
    <created-at type="datetime">2010-07-25T23:00:13-07:00</created-at>
    <deal-id>378601</deal-id>
    <deal-image-url nil="true"/>
    <deal-url>http://www.mycoupons.com/jump/coupon/378601/1065</deal-url>
    <description>Send Dad a Gourmet Gift Cake! and save 10%</description>
    <end-date nil="true"/>
    <free-shipping>no</free-shipping>
    <id type="integer">196535</id>
    <image-tagline nil="true"/>
    <keywords>None</keywords>
    <last-updated>2010-04-20 03:01:54</last-updated>
    <merchant-display-url>http://click.linksynergy.com/fs-bin/click?id=iB4OSEnAAQw&offeri...>
    <merchant-id>9635</merchant-id>
    <merchant-image-url>http://media.mycoupons.com/advimgs/medium/9635.gif</merchant-ima...>
    <merchant-name>Bake Me a Wish</merchant-name>
    <merchant-url>http://click.linksynergy.com/fs-bin/click?id=iB4OSEnAAQw&offeri...>
    <rating type="integer" nil="true"/>
    <search-domain>linksynergy.com</search-domain>
    <site-wide>no</site-wide>
    <start-date>2007-05-31</start-date>
    <title>Send Dad a Gourmet Gift Cake! and save 10%</title>
    <updated-at type="datetime">2010-07-25T23:00:13-07:00</updated-at>
</coupon>