For the offline tools that support multiple inputs, the best I've seen so far is https://github.com/wolverdude/GenSON/ I'd like to see a tool that takes filenames on standard input because I have thousands of files. However, I run out of open file descriptors, so make sure the files are closed. I'd also like to see JSON Schema generators that handle recursion. I am now working on generating Java classes from JSON objects in hopes of going to JSON Schema from my Java classes. Here is my GenSON script if you are curious or want to identify bugs in it.
#!/bin/sh
ulimit -n 4096
rm x3d*json
cat /dev/null > x3d.json
find ~/Downloads/www.web3d.org/x3d/content/examples -name '*json' - print| xargs node goodJSON.js | xargs python bin/genson.py -i 2 -s x3d.json >> x3d.json
split -p '^{' x3d.json x3d.json
python bin/genson.py -i 2 -s x3d.jsonaa -s x3d.jsonab /Users/johncarlson/Downloads/www.web3d.org/x3d/content/examples/X3dForWebAuthors/Chapter02-GeometryPrimitives/Box.json > x3dmerge.json