I really like how they break down the data for you here. There are more and more ways of looking up salary data, but many of them don't allow you to isolate for things like years of experience, or give you a market rate as well as the median rate, and many of them don't seem very reliable. To me, Glassdoor has become reputable enough that I'd feel comfortable using this tool to negotiate salary. I'd guess it's not as valuable yet for people who aren't working in tech, but for my title/position (Product Designer), the sample size is large enough to provide a high degree of confidence.