This is a plugin for TileMill that allows you to open a JOSM editing session based on the current map view (JOSM must already be running). It adds a button to the top left corner of the map preview at zoom level 14 and above.
It can be helpful in the development of an OpenStreetMap-based TileMill project for quickly checking and fixing issues with OSM data as you come across it.
The plugin should be available from the Plugins panel in TileMill. You can also install it manually by cloning this repository into your TileMill plugins directory.