While definitely not a comprehensive test suite, here is a page I had set up ages ago when comparing JSON decoders:
http://gggeek.altervista.org/sw/article_20070425.html
What I was especially interested in are "edge cases", ie. strings which fall outside the spec but might be decoded as something else than NULL by -some- libs at -some point in time- (ie. comments, trailing commas etc), or generate errors/warnings which one could also like to keep stable as part of an API contract.
That's why I think it might still be useful, as it seems that hoaproject focuses on parsing *valid* strings instead.
Bye
Gaetano