JSON viewer for OS X
Jsoneditor is a browser-based editor that you can deploy locally or use the ad-sponsored online version provided by the author. It's handy for small JSON files, and I find it the easiest and most feature-full. It will highlight broken json, but it's not a 100% compatible validators like https://jsonlint.com/ because, for example, it treats raw tabs as valid within strings although this is not allowed by the JSON spec. 

JSONMate seems to be a usable open-source project with an online editor which probably won't be able to handle large files, but you can fork it on github, adapt to your needs and run it locally.

Intellij Idea and derivatives have decent JSON support but large files take time to load.

SublimeText is also usable for JSON if you install the plugins for beautifying and extra features.

For windows XMLMarker does a good job as an XML and JSON visualizer, but the last version which added JSON support is a limited trial.

Disclaimer: I am not affiliated with any of these tools and projects.