This topic seems to be already popularly discussed a little more broadly in another question: 
[In which countries are academic salaries published?](http://academia.stackexchange.com/questions/26487/in-which-countries-are-academic-salaries-published). 


That discussion seems to focus less on online resources, but nevertheless provides a few good examples:

 - [Annual report on faculty salaries](http://data.chronicle.com/?cid=megamenu#id=table) produced by the AAUP (through The Chronicle of Higher Education)

 -  @Christian Clason mentions a promising database containing [Public University Salaries](http://notable.math.ucdavis.edu/wiki/Public_University_Salaries).  


An additional good online resource that I've found = [GlassDoor.com](https://www.glassdoor.com/index.htm)

Know any others??