PhotoTranslator is a neat application for Nokia N900. As the name says, it allows you to translate text from pictures to various languages.
Imagine yourself walking on a street somewhere in a foreign country and you face a warning sign, but you have no idea what it says. Just take a picture of the sign and open it with PhotoTranslator, crop the text in the sign and PhotoTranslator translates the text to your language.
PhotoTranslator can be used also for translating copy/pasted or typed text, not just for text from pictures.