Encyclopedia and Reference Sites
Wikipedia, "the new kid on the block", as of March, 2016 had approximately 38 million articles in 253 languages, with more than 5 million English language entries.
While there have been some questions as to its reliability, it provides a quick and easy first take on literally hundreds of thousands of topics.
Britannica now provides on-line (for free!) the complete, updated Encyclopedia Britannica, the oldest and largest general reference in the English language, plus selected articles from more than 70 of the world's top magazines and links to 1000's of sites.
Richard Jensen is a Research Professor of History at Montana State University. He has compiled an extensive list of general and topic specific resources in the Humanities & Social Science areas.
Encyclopedia.com takes a different approach than Wikipedia. Rather then community editable entries, they have compiled information from more than 100 trusted sources, including encyclopedias, and dictionaries.
Rather than just dismiss Wikipedia, some instructors have helped improve it by assigning students to either write an original Wikipedia article or do a major edit on an existing one.
Nothing focuses a students attention like the knowledge that their work will be viewed and reviewed by people from all over the world.
If a lesson is devoted to researching multiple items, show your students how to open two browser windows side-by-side. The research site(s) can be loaded in one window and the stimulus material in the other window.