Out of the hundreds of tools for astronomers available, I find that I keep coming back to a handful of tools that I find most useful.  Some are free (with ads) and some pay.  Here are some of my favorite smartphone apps:

Sky Safari Planetarium (Android, iOS)

A good planetarium app is a must for the novice and most experienced astronomer.  My most used features in the premium version are the Field of View Indicators (set up your equipment under “Observe”, then setup the FOV under “Scope Display”.  City dwelling astrophotographers will especially appreciate the “H-alpha” display in the “Milky Way” display setting.  This shows the relative brightness of the narrowband objects in the night sky.  With additional hardware, you can use this app to control your GoTo telescope mount.  There are too many useful features to list here.

Stellarium Planetarium (Android, iOS)

This app runs on all the popular platforms and is a worthy contender to Sky Safari.

Clear Outside (Android, iOS)

This handy app from First Light Optics in the U.K. is indispensable for predicting the sky conditions on up to a week in advance.

Weatherbug (Android, iOS)

A second, less astronomy centric, opinion of weather conditions, and a live cloud cover map, and generally accurate humidity predictions.  Knowing the humidity in advance helps me prepare for dew.  In my experience, any humidity over 75% has the potential for dew.

Windy (Android, iOS)

A third opinion, and the coolest animations.

Meteoblue Astronomical Seeing (https://www.meteoblue.com/en/weather/outdoorsports/seeing)

A fourth opinion and includes the predicted astronomical seeing conditions in arc-seconds.

Storm Surfing (http://www.stormsurfing.com/cgi/display_alt.cgi?a=nam_250)

This shows the jet stream predictions several days in advance.  The jet stream affects seeing.

Space Weather Live (Android, iOS)

This app shows live data from NASA and other sources of the sun in several wavelengths.  It is wonderful for knowing if there are any sunspots or prominences without having to set up the telescope.

ASICAP (Android)

For this app, you will need an OTG (On The Go) cable to connect your ZWO camera to your smartphone.  I find this very useful for checking focus when the computer is not close enough to the telescope.

Bahtinov Mask Generator (https://satakagi.github.io/tribahtinovWebApps/Bahtinov.html)

This lets you generate an SVG file so you can laser cut your own Bahtinov focus mask.