The Pew Research Center is an American "fact tank" based in Washington, D.C. that provides information and data on issues, attitudes and trends shaping the United States and the world. It was established in 2004 as a nonpartisan subsidiary of Pew Charitable Trusts.
The group's Journalism.org site focuses research on public opinion and issues within news media. It's research is often cited in media bias research done by AllSides, including the 2014 Where News Audiences Fit on the Political Spectrum study.
Sources: Pew Research and Wikipedia