The Dictionary app that Apple forgot.
While iOS has an excellent built-in Dictionary service, there is absolutely no front-end way to access to this functionality. This app is a simple wrapper app to the built-in dictionaries, and as such there is no overhead on storage unlike other dictionary apps. It is also completely useable without Internet access, as long as the dictionaries have been downloaded already. By default only the native language is installed, but it is possible to add download languages as well as translation dictionaries through the manager.