Smithsonian's History Explorer - Use this resource from the Smithsonian Museum to explore American History!
Mystery Science provides free interesting lessons and videos on a variety of different scientific concepts. You can sign up for mini-lessons to go along with each video.
Crash Course Kids YouTube channel provides videos on almost every science concept covered in K-5. I cannot recommend this channel enough!
BrainPop provides videos, lessons, games, and activities for multiple subjects.
Other recommended websites and virtual tours!