In the interest of being "old school" here is a list of my favorite sites and I will tell you right now do NOT waste my time asking me to include your site. I hate that. If you want your site listed here then make it really good. Don't give me any crap about "links are important".
General Interest Websites
Aeon Magazine - This is a digital magazine and it's a great site to browse if you have time to waste.
The Perspectist - A Website that publishes listicles.
Top Ten Topia - Lots of great articles about every topic you can imagine.
Undark Magazine - This site is loaded with interesting science articles.
Wired Magazine - This is the digital version of the great print magazine.
History Websites
Edge Adventures - Not really sure what this site is supposed to be but it sure takes you everywhere.
Medievalists.Net - Covers all things medieval, in case you are not sure.
National Archives - These are the national archives of the United States. Lots of history here.
Science Websites
Popular Science - A great magazine when I was a kid, the Website is still pretty good.
Science Daily - About all the science news you could ever need.
Science Daily - About all the science news you could ever need.
Science News - Just what the title says and nothing else.