Think tank is a phrase used to describe organizations (usually non-profit) which conduct public policy research and analysis.
They often publish papers based on their research and make them available for free on their web sites.
Not all think tanks are non-partison. Be sure to take this into consideration when evaluating their material.
The Think Tank Spectrum in Extra!, a publication of FAIR (Fairness and Accuracy in Reporting) compiled a list categorizing some of the most frequently cited think tanks as conservative, progressive, centrist, etc.