Napkin is a Mac-only app that stores and syncs its documents on iCloud, Apple's ubiquitous online service. It has no iOS version. Nothing for iPhone. Nothing for iPad. Yet, with iOS 7, you can now drill down into Settings > iCloud > Storage & Backup > Manage Storage, select Napkin, and delete any of its saved documents with a single swipe and two taps. Same holds true for other Mac-only apps, including Apple's own TextEdit.
That's both kinda cool... and kinda scary.
Update: Looks like this worked on previous versions of iOS as well. Thanks, Sarah!