Coordinate Master is one of the most powerful map coordinate converters available. It uses the PROJ4 library and a lookup file containing projection and datum parameters to allow conversion between more than 1700 coordinate systems from around the world.
Lat/lon, UTM, US coordinate systems (including US State Plane), Australian coordinate systems (including GDA2020), UK coordinate systems (including British National Grid) and many, many others are supported. Custom coordinate systems can be added and affine parameters are supported for adding local grid systems.
The app can use manually entered coordinates, your current GPS location, or get the lat/lon coordinates from a photo.