A Good Translator
Few things can make you feel more helpless in a foreign country than not being able to communicate. Improvised sign language only gets you so far. Most language translation apps are far cheaper than printed phrase books, and they usually carry much more information.
Lonely Planet's phrase book apps each cover a single language and cost a mere $6. These handy tools take advantage of Lonely Planet's wide experience in the world travel market. Taking the time to memorize a few phrases will probably earn you points with the locals.
For a cheaper app that translates using voice to voice (you speak in one language and it repeats it in another), iTranslate is your best bet. If you don't mind seeing ads, iTranslate language titles are free, or you can upgrade to an ad-free version for $1 to $2. Jibbigo is another translator that's very highly rated, but will cost you around $30.00 per language -- not exactly budget-conscious, especially if you need multiple languages.
Even if you use the on-the-fly translators, you'd still do well to memorize some important phrases. Considering how shaky some of the translator apps are, knowing how to say "I'm sorry, I didn't mean to say that!" might come in handy.