Close seconds are salary studies in printed publications.
Knowing your fair market value is the centerpiece for negotiating the compensation you deserve. Market value is the going rate for people in your industry with skills and a job description similar to yours. Most people guard their income figures like gold at Fort Knox, but a smooth talker like you may be able to pry loose pay figures by promising to trade earnings information once you find a job. In gathering salary data from surveys, the mouse may be mightier than the pen. Here is a small section from a virtual fountain of resources.