Non-Governmental Organizations (NGOs) are generally not-for-profit groups and not affiliated with any government entities.
Intergovernmental Organizations (IGOs) generally consist of representatives from soveiergn states pursuing common goals established by a formal agreement. See some sample IGOs below:
"Think tank" is a phrase used to describe organizations which conduct public policy research and analysis. they often publish papers based on their research and make them available freely on the web. Note: some think tanks are nonpartisan, but others advocate particular political positions. Be sure to take this into consideration when evaluating their material.