JSON Validator: https://jsonlint.com/
JSON Online Editor: https://jsoneditoronline.org/
JSON string escape/unescape: https://www.freeformatter.com/json-escape.html#ad-output
JSON Diff: http://www.jsondiff.com/
JSON to CSV converter: https://json-csv.com/
JSONItem_MTC: Drop in replacement for the native Xojo JSONItem https://github.com/ktekinay/JSONItem_MTC
All these things helped me work with/debug JSON in one way or another and were helpful to me.
Hopefully they can be helpful to others as well.