This is one of the most practical uses of augmented reality I've seen. Word Lens translates text you capture from an iPhone's camera to another language. I think it's only Spanish-English/English-Spanish at the moment, and $4.99 per language.
If you're skeptical, as some YouTubers seem to be in the comments, you can see an example of the translation I did on my phone of an English magazine page to Spanish. Works best on big text, and probably most practical for signage. I need this for trips to Italy (my Italian is rough), so hopefully Italian will be available in the future.