So I noticed that the current website for checking a theme (not the icekirby one, but the JSON one) kinda tells you jack about where the errors are. SO I went out and found this baby http://jsonformatter.curiousconcept.com/ It allowed me to check all my horrid errors with commas and the like and told me WHERE THEY WERE (Room gasps here). So if someone else already posted about something like this I apologize, but I'm not reading through all the threads to find it.
I think http://www.jsonlint.com is better, due to the fact that they are very clear and they show you where your errors are.
Curious concepts does that too with more clarity. :P jsonlint is just faster for small themes, whereas the formatter is better for the larger, clunky themes. It also picks up more hardcore mistakes.
I just use Notepad++'s JSLint plugin. It's faster than both (specially because I can use a shortcut key instead of copying/pasting), although it's not as clear when explaining the error for JSON stuff. Of course, that is irrelevant if you actually know what you are doing with your JSON file.
I've posted this site a few times around the section. Its also on the documentation page on the wiki. The main use of it is the "Invalid Array Length" where jsonlint grinds to a screeching halt. Other than that, the only advantage to the link you posted is that it shows most of your errors (and some false positives due to other errors) at one time, instead of error by error.
Most of the times it's just a missing comma after a string or an object so when it says what line the error is on 95% of the time I can manually solve the error without learning what the actual hell it means, though the wording was a little confusing when I first started creating my first theme.