This is not a widget, it toggles the setting and quit immediately without staying in the memory, so if you're looking for an app that doesn't occupying lots of memory, this is a good choice.

This version is a new version of following app:
https://play.google.com/store/apps/details?id=com.dqtools.brightauto

It contains two improvements:
1. Avoid popup an empty screen when switching auto brightness. (Thanks for the suggestion from Fabian)
2. Auto save current manual brightness level. If you switch on and off auto brightness again it will stay in the previous level. In some devices it might require permission to modify settings. (If you don't like this feature you can still use old version which link can be found above.)