Mobile Tools by AudioControl
It is marketed as a Car measurement app that includes speaker polarity, but you can use it out of the car as well.
The basics of SPL (sound pressure level) and RTA or FFT for frequency response measurement is what is most useful in most situations like placing speakers, equalizing the room, analyzing feedback and keeping an eye on healthy sound pressure levels.
The Audio-Technica Installed Sound Support App
This free app is intended for sound contractors and front-of-house technicians. Along with RTA with SPL display it includes a wavelength calculator, a wireless mic calculator as well as mic gain and cable calculators