I find the National Security Archive, based at George Washington University, fascinating. The entity provides many services including an “investigative journalism center, research institute on international affairs, library and archive of declassified U.S. documents… and indexer and publisher of former secrets.”
Their website is place some individuals could spend hours. Researchers can peruse a list of recent postings and documents from the homepage. There is also a link to their blog, UNREDACTED.
Searchers can also follow the “documents” link to find the Electronic Briefing Books. These briefing books, arranged by topic, provide access to a small portion of their collection of declassified documents.
The National Security Archive also offers access to thousands of other documents via a paid subscription (or visit your local university library). To learn more about that, check out the “research” tab.