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.
Some think tanks are non-partison, but others advocate particular political positions. 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.