Take a picture of text in a foreign language and have the text translated to another language, on the convenience of your iPhone.
At the beginning of TreeHacks we were inspired by the possibilities IBM's Watson API had to offer for language processing. We integrated ABBYY's OCR API into our app and then started working with IBM's Bluemix Watson API to translate text. Unfortunately, the Watson Translation API has a fatal bug in which the translation returns "TranslationError" for the entire translation if an unrecognized word is found. We then went to Microsoft's Bing Translator API and got that up and running. We are most proud of the OCR functionality and its accuracy.