This topic seems to be already popularly discussed a little more broadly in another question: In which countries are academic salaries published?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 produced by the AAUP (through The Chronicle of Higher Education)
@Christian Clason mentions a promising database linking Public University Salaries.
An additional good online resource that I've found = GlassDoor.com
Know any others??