When you're traveling your mobile phone sometimes becomes the most important item you packed. It's your gateway home, it's your research platform, it's your jukebox, it's your communication center and so much more.

Be sure to pack your external battery because here are 25 essential apps you'll need as a traveler.

The apps are packed into ten neat categories and are linked to the Apple Store and/or Google Play Store.

Accommodation

Because everyone has to sleep somewhere

• Airbnb - available for iOS and Android - Probably the best way to get in touch with locals is to sleep at someones place for a small fee, Airbnb is your community to connect with amazing people worldwide.
• Couchsurfing - available for iOS and Android - Sleep on someones couch for free and get in touch with locals with Couchsurfing.
• Hostelworld - available for iOS and Android - Your search engine for the hostels which are most trustworthy, clean and friendly.
• Booking.com - available for iOS and Android - No hostel, no Airbnb and no couch available? Or do you want some privacy? Booking.com has a huge selection of hotels with special deals just one click away.

Transportation

Because you want to get from one place to the other

Be sure to check out the many local alternatives, e.g. 511.org for public transportation in the Bay Area, Wanderu for bus and train travels in the US, HyperDia in Japan, DB Navigator in Germany, New York MTA Subway Map in New York, OmniLíneas in Argentina, Busportal in Peru and so on and so forth.

• Uber - available for iOS and Android - Need a ride? Uber is now available in several countries and usually a safer and cheaper alternative than a cab.
• Lyft - available for iOS and Android - Lyft is similar to Uber but currently operates only in the US.
• Skyscanner - available for iOS and Android - Get the cheapest flight deals with this search engine, but be sure to check out its competitors Google Flights and the Matrix Airfare Search as well.

Navigation

Because you want to know where you're going

• Google Maps - available for iOS and Android - Google Maps offers you navigation at the tip of your fingers but requires an internet connection, though there is a function available to download certain map data.
• OsmAnd - available for iOS and Android - If you're stuck without an internet connection you might as well opt for OsmAnd, it offers you offline access to OpenStreetMap once you've downloaded map data, don't be fooled, this could save your life at some point.

Translation

Because sometimes you don't speak the language

• Oanda Currency Converter - available for iOS and Android - Currencies may not be the language you've thought of, but they need translation as well. This app by Oanda, once installed, caches the current exchange rates so that you can access them offline as well.
• QuickDic - available for Android - QuickDic gives you offline access to your dictionaries. Simply store the desired files on your memory card and have them handy when you're on the road without an internet connection.
• Google Translate - available for iOS and Android - The app by Google offers a lot of different languages and comes with a function where you can use your camera to translate text instantly.

Bookkeeping

Because otherwise you live beyond your means

• Toshl Finance - available for iOS and Android - Every traveler is concerned with how much he spends per day for what, this app helps you watch your daily limit, it supports multiple currencies and tags.

Recommendations

Because the community is powerful

• TripAdvisor - available for iOS and Android - Ever been in a city without a travel guide? TripAdvisor comes in handy when you're looking for a hotel, restaurant or a sight. Sign up and you can leave comments and rate locations as well.
• Foursquare - available for iOS and Android - Your handy city guide, comes with an automatic location tracking which gives you suggestions based on where you are, it requires an internet connection though.

Communication

Because you want to stay in touch

• WhatsApp - available for iOS and Android - Stuck in a foreign country with no speech and no text service? Connect to your Hostel network and start messaging via the internet with WhatsApp, it has never been easier to stay connected with your friends at home.
• Google Hangouts - available for iOS and Android - Video-telephony made easy, after months away from home you can brag about how tanned you've become.
• Skype - available for iOS and Android - Another video-telephony app to keep you connected with your family and friends.

Read

Because sometimes you're on long bus or train rides

• Kiwix - available for iOS and Android - Kiwix is your offline Wikipedia. Once you've downloaded the data files, which can be quite big, you can access the articles without an internet connection.
• Moon+ Reader - available for Android - A neat and handy small app so you can access the tons of Lonely Planets you bought. A good alternative to an ebook reader and paper books.

Spread the Word

Because at some point you want to show off with your travels

• Instagram - available for iOS and Android - Take photos, apply filters and share them with your friends, show me a traveler who doesn't like to take pictures!
• Twitter - available for iOS and Android - Compile your thoughts into tweets and share them with your fellow travel colleagues.
• Facebook - available for iOS and Android - All those friends who are at home want to have the latest updates on your travels.

Nerd Stuff

Because a world without nerds would be boring

• Flightradar24 - available for iOS and Android - Travelers love planes, why not follow them on your phone? Launch Flightradar24 and see the air activity in your proximity.
• Google Sky Map - available for Android - Ever looked at the sky and asked yourself which constellation you're looking at? No more guessing necessary with this handy app by Google.