Think-tanks

Nonpartisan

Brookings Institution [W]: "Brookings describes itself as independent and non-partisan. A 2011 study examining think tank employee donations from 2003 to 2010 showed that 97.6% of Brookings's employees' political donations went to Democrats and described the think tank as "liberal." Yet, a 2005 academic study concluded it was centrist in that it was referenced as an authority almost equally by both conservative and liberal politicians in congressional records from 1993 to 2002."
New America Foundation [W]: "public policy institute and think tank focusing on a wide range of issues, including national security studies, technology, asset building, health, energy, education, and the economy" Chair of Board of Directors is Google's Eric Schmidt.[1][2]

Analyst Institute (web site): "a clearinghouse for evidence-based best practices in progressive voter contact."
Center for American Progress [W]
Institute for Policy Studies (left of center)
Nation Institute (web site)

Note: some of those on the Americonservative list are also in this category.

Rockridge Institute [W]: think tank founded in 1997 and located in Berkeley, California from 2003 until April 30, 2008; "non-profit research and progressive" according to Wikipedia, but "non-profit, non-partisan" and progressive according to at least one editorial

2012-06-01 Think Tank Spectrum Revisited: Conservatives gain within still-narrow spectrum
1998-09-01 Heritage Clones in the Heartland "Local think tanks' "research" comes pre-digested"