As we write this (fall of 2016) nearly all US governmental bodies have extensive web pages crammed full of tax information.  Here we will provide some of the tax and financial information links that we find most useful or interesting.   

US Treasury Home Page
Guidestar - Non-profit Organizations tax returns and data base
New York State Department of Taxation Home page
The AICPA's CPA Links (Lots and Lots of tax and Accounting links, including all states)
New York City Department of Finance Home Page
American Insittue of CPAs