Hey, even if Microsoft sees fit to create a Groupon client for all the other platforms except their own, at least we still have 3rd party developers.
Groupon Express by Fast Code is a Groupon client for Windows Phone 7. It allows you to browse the daily deal provided by Groupon, and allows you to search the deals information by your location.
The app also neatly provides a feature to navigate you to the store you choose. You can send the direction information to other people by email or messaging, and you can store it in the phone memory so that you can conveniently find the store you want to go to when you cant connect to the network.
The app is $0.99 with a free trial and can be found in Marketplace here.