VisaDoor is a great resource, and we recommend it pretty heavily at http://OfferLetter.io .

There's something very visceral about being able to look at someone's specific offer, knowing that there is a person out there

I also wrote a bit about salary research in general, on our blog:

http://offerletter.io/blog/201410-research.html

And yes, to your point - absolutely more accurate than Glassdoor.