While there are tons of online resources that help you plan your trip, it's always a great idea to have offline back-ups. A print out of the map, copies of your documents and local currency are definitely worth carrying (ATMs can be few and far between at most places).
I've also found it's useful to keep copies of addresses of your hotels, contact numbers of local guides to name a few in a little notebook in case technology fails (and it will because, Murphy's Law!) :)
What are some of the go-to apps you use for your travels? Would love to exchanges notes!