Whenever we discuss deal of the day or group buying sites, aggregator AllTheDeals usually rates a mention in the comments. The site now has an iPhone app to let you track deals from multiple sites on your device.
AllTheDeals tracks deals from Catch Of The Day, Daily Offer, 1-Day, Scoopon, Streets, Zazz and others. If you're not sporting an iPhone, the RSS feeds for different deal categories might be appealing for mobile use.
All The Deals is a free download for iOS devices. (A premium ad-free $1.99 version has been developed, but isn't showing up in the iTunes Store as of this writing.)