"Must have" is hard for me to apply. But these are the ones that are on my home screen .
Row 1 :
Self Explanatory
Row 2 :
Weather
AeroWeather is a great aviation based weather site great detailed near real time weather info
Maps
Airports is a directory from AOPA of small airports across the US. Great places to ask about pitching a tent, I have been offered empty hanger bays.
Row 3 :
Hosteling International App for finding and booking stays
Skype
WordPress is whom I use to host my blog. I find it easier to let them be the webmaster while I'm on the road.
WarmShowers I have not used this yet as I just joined late last year but it belongs on my home page I'm sure
Row 4 :
Kindle reading app for accessing my books
Pandora for music/relaxing when I have a wifi signal where I am staying
GreenDot is a "throw away" debit card, I only keep a few days budget on it at a time. If I lose it no big deal you can buy another any WalMart, Drug stores, register the replacement number online move the balance to the new card and keep going. No bank to notify etc. If someone finds the old one in the mean time they still need to guess the PIN number to use what ever is left on it anyway.
Sunrise is an almanac to let me see the sunrise/sunset times for the area I am in. Good for planning the morning pack up and go routine.
Row 5 :
Self Explanatory