If I want to do any quick fact-checking, my favorite online source is Wikipedia, a free encyclopedia. Many librarians will frown on my choice of sources, since anyone (gasp!) can create or edit Wikipedia entries. To put a more positive spin on that fact, the Wikimedia Foundation says, "Imagine a world in which every single human being can freely share in the sum of all knowledge. That's our commitment."
My qualms with relying on Wikipedia for factual accuracy were explored (and assuaged) by this New York Times Magazine article. See what you think! (If you have trouble accessing the article via the link, make a quick comment and I'll explain another way to find it.)