HHPanningTableViewCell is a UITableViewCell implementing "swipe to reveal" a drawer view. Such a view typically holds action buttons applying to the current row.
This behavior is seen in a number of iOS applications. To my knowledge the idea was pioneered by Loren Brichter for Tweetie (aka Twitter for iPhone).
The HHPanningTableViewCell implementation was written for the ACTPrinter 4.0 application. The code presented here is identical to the one used in the shipped product.