You don't want to be lost in a foreign country so I suggest have all of these. Just in case
You can never be too careful. Check out my customs and laws list per country for more.
Whether its communication with locals, other travellers or friends and family back home. You don't want to be isolated while abroad.
Manage your money overseas! Check out my Travel Card section as well :)
These ones aren't essential but are useful extras you can download if you'd like!