A bubble/spirit level tool, with both a bull's eye level and horizontal/vertical levels, as well as a compass (showing both true/geographic and magnetic north), and support for the Apple Watch. Watch support allows you for example to set your iPhone on the top of a tripod, and work on leveling its legs by looking at your watch - without having to go through the stand-up-to-look-level/crouch-to-adjust-leg routine.

The tool is normally included in my astronomical app (Polar Scope Align Pro), but I decided to release it as a separate app because I could not find anything with the features I wanted on the app store.
You can try the free ad-supported version that lacks the Watch support.

Basic Features:
-Bull's eye level with surface angle reading, horizontal/vertical levels with angle readings (0.1 degree steps).
-Compass with true/geographic north and magnetic north heading readings in degrees and direction.

Extra features:
-Apple Watch support.
-Magnetic declination reading, which shows the offset between magnetic and true north for when you want to use a physical magnetic compass (requires access to your location).
-Accelerometer/Gyro switch in case one of the two works better on your phone (Gyro is default).
-Gyro calibration to compensate for lens bulges or uneven cases when using the phone as a bubble/spirit level.
-Gyro rotation rate function: measure instantaneous or cumulative rotation on 3 axes (pitch/yaw/roll).
-Magnetometer (compass) calibration level display.
-Red-color night mode to preserve night vision.