Sources of academic salary information?
The broadest survey that I'm aware of across many disciplines in the US is the CUPA-HR (College and University Professional Association for Human Resources) survey.  Some summary data is available for free from their web site.  However, you'd have to pay for access to the full survey results.  Colleges frequently make use of this survey in setting salary targets. 

The Chronicle of Higher Education also hosts a database that has salary data for faculty at many individual institutions in the US- I'm not sure of the sources of this data.  

Many individual disciplines have salary surveys that are done by professional societies.  For example, in mathematics, the American Mathematical Society has an annual survey of salaries of mathematics faculty in the US with extensive results that is freely available on the web.  The survey breaks down positions by type of institution (bachelor's granting, master's granting, and three classes of PhD granting institutions.)  It also has data for new assistant professors, all assistant professors, associate professors, and full professors.    

At many public institutions in the US the salaries of faculty are a matter of public record and can be obtained by doing a simple web search.  In negotiating an offer you might find this data useful.  However, it's quite common under these circumstances that the institution simply won't negotiate much with respect to salary to avoid upsetting the faculty that already work in a department- they're generally aware of what their coworkers make.   

At private universities in the US, the salaries of individual faculty are generally kept secret, except in cases where faculty members are among the most highly paid employees of the institution whose salaries must be reported as part of the annual report to the IRS (typically the five highest salaries go to football and basketball coaches and top administrators, but you sometime find medical school profs listed in these reports.)