Microsoft is always looking for showcases for their new Pinned Site technology in IE9. Facebook is already a pretty good example, providing notifications for example, and now Groupon has also signed up, providing a great experience for deal seekers.
In the promotion the first 100,000 IE9 users who pin the Groupon site to their task bar will get 5 Groupon Bucks which can be redeemed on the site. Winners will be notified by a banner on the site.
Read more about this promotion at the Windows Team Blog here.