Guidestar http://www.guidestar.org/ has been a site that I have come back to multiple times in the past couple years. Every non-profit must fill out a 990 tax form. At Guidestar (you must register but it is free) you can search for any non-profit company and look at its 990 form.
While there is obviously large amounts of useful information for art administration jobs and theatre managers, why is this useful to TD's? There are several reasons. First, the forms includes some salary information, which can be helpful if you are looking for a new position, or to provide insight into your own organization. Secondly, I thought it was interesting to compare my theatre company with others that I was associated it with. It is also an interesting check, as the financial side of a company looks very different sometimes from the financial paperwork. And, lastly, since multiple 990 forms are on the site you can look at trends (are they gaining or losing money, are salaries going up....)