Grey literature may be very important for a number of topics; these are articles, reports, etc. from various organizations, individuals, etc. that are not the typical "peer-reviewed" literature. However, this literature is oftentimes extremely well researched & cited (although again, it will be difficult to locate the "paper trail" for these works). To the right, you will find some recommended places to start for this literature. One other caveat involves working papers or conference papers. It is usually best to determine first whether these papers have been published before using the working/conference paper version, since these are usually "drafts" of a more revised article.
There are numerous public policy institutes and research programs across the country that provide insightful analysis of numerous public policy/affairs issues. The Kennedy School of Government at Harvard also created a custom Google search engine for searching this literature. Note that you can also create your own search engines if you already have a few sites identified as relevant sources of information.
For international relations and foreign affairs, first check out CIAO--clunky interface, but good content.