Long time user of Meld as soon as I need to compare folders/multiple files... but Differ’s way to handle JSON looks very interesting, thanks for sharing this! I’m going to give it a try in the next days ;)
For files (not folder), another great one I ’ve been using for years is P4merge , from Perforce/Helix. The merge/conflict tool is free (not open source though), works on Win/Mac/Linux and does the job really well. (With great support for 3 panes diff/merge, when you need to compare revA vers revB versus their common ancestor.... really awesome for conflict resolution!)