Generally I use vi to create all my .json files (because the Mac is notoriously annoying about mangling file extensions for developer-type files). I use an online JSON editor like the one BDub suggests, then I just copy the file to my clipboard and do:
a (puts you in append mode)
[esc] : wq! (saves the file)
I know it’s ugly and low-level, but it’s one of the most reliable ways I know of editing JSON and not letting editor crap get stuffed in there. For example, if you use TextEdit, and you’re typing quotes, it will often use the directional quotes (left-slanted open quote and right-slanted close quote), which are unicode characters and will cause a JSON parser to choke.