Lonely Planet This app comes with two guides. A guide for San Francisco and a Mexican Spanish language guide, complete with spoken pronunciation. I used this app so many times it's not even funny. The spoken part of the app is fantastic as it's slow enough for you to learn the words which is good as it's unlikely you'll want to whip out your phone in every location. Oh, and it works offline.

Dropbox This app was a life saver. I don't want to carry around travel guides and often I'm only using them when I'm at my hotel planning the next day's travel. By downloading a guide from Lonely Planet and putting it into Dropbox I was able to use my guide whenever I wanted, online or not and it didn't take up more space in my backpack.

Google Translate Not super useful since you must be online to use this however you can save phrases to use later (offline) and when you're at a hotel with wifi it can indeed help you get at some of the finer points of what is going on since you can translate right then when connected. That said, the spoken part of the app is often hard for native speakers to understand and even harder for you to learn as it's quicker than I'd like to see. Still, looking up one word can help you learn what you need and often that one word can substitute for a complete sentences. Definitely worth having and used.

Skype While the internet in Nicaragua isn't awesome I did still manage to use this app to call home a few times (and to check my personal voicemail since I had my phone in airline mode). The app version isn't quite as nice for calling foreign numbers as it always tries to call US but with some use of your notepad you can put in foreign calling codes.