Is there a minimap plugin(function) in QGIS?
In addition to the brilliant suggestion by @Jakob, there is the Auxiliary Window plugin which opens up a new window showing some basic map features such as showing/hiding layers, adding/removing layers from the main window etc:
Similarly to the Dockable MirrorMap plugin, the Auxiliary Window also allows for multiple windows to be loaded.
Hope this helps!
Overview will show any layers you add to it. What is it you're looking for in a minimap plugin that built-in overview does not provide?
There is a plugin called 'Dockable MirrorMap' that will give you an extra map, where you can control layers shown.