I have also used Booking.com in Greece to help me find last minute lodging.Other than posting to the mobile version to this website while in Greece to ask our community advice I would say the following travel apps would work well for Greece and just about anywhere:
1. Google or Apple maps (need to know where you are LOL).
2. Google translate (for non Greek speaking people this can help with basic communication).
3. Trip Advisor App (really is the best travel app and have personally used it in Greece).
Hope this helps!