The Ultimate STEM Resource Guide for Kids By ComputerScience.org:

Explore the best online learning resources for kids of all ages.
https://www.computerscience.org/resources/stem-for-kids/

How Stuff Works:
A source for clear, reliable explanations of how everything around us actually works.
http://www.howstuffworks.com/

Brain POP™:
Educational animated movies for K-12 created to help explain concepts and to demystify math, science, health, technology, and English topics in an entertaining way for both children and adults.
http://www.brainpop.com/

NASA:
Enter the NASA portal to explore the many facets of NASA. Learn the latest in space news; find math, science, and technology activities for home or the classroom; shoot a cannonball into orbit; or design your own mission to Mars. It’s all just a few clicks away.
http://www.nasa.gov

Imagiverse:
Imagiverse.... the universe of our imaginations has no boundaries. Imagiverse believes that imagination is the key to learning. Teachers can teach, but learning comes from within. The web site's goal is to spark the flame that will lead young and old to pursue learning as an adventure through imagination. Activities, resources, and lots of information are available for all to see and use.
http://www.imagiverse.org

The Library of Congress

More than a library. . .an online collection of the words, pictures, and sounds of our country in a library that never closes. http://www.loc.gov

NASA:
Enter the NASA portal to explore the many facets of NASA. Learn the latest in space news; find math, science, and technology activities for home or the classroom; shoot a cannonball into orbit; or design your own mission to Mars. It’s all just a few clicks away.
http://www.nasa.gov

The NASA SCI Files™
Come explore the tree house and help the tree house detectives solve their latest problem. Conduct experiments from Dr. D’s lab, play the online Problem-Based Learning (PBL) game, or just check out the research rack, expert’s corner, or media room. This is a great site for parents, educators, and students in grades 3-5.
http://scifiles.larc.nasa.gov

Association for Women in Science (AWIS):
An organization dedicated to achieving equity and full participation for women in science, mathematics, engineering and technology.
http://www.awis.org

The JASON Project:
Connects students with scientists and researchers in real- and near-real time, virtually and physically, to provide mentored, authentic and enriching science learning experiences. JASON and its partners create these connections using multiple platforms and technologies, including award-winning, standards-based classroom curriculum developed with NOAA, NASA, National Geographic Society and others; after-school and out-of-school activities; camp experiences; and exploration programs for museums, aquariums, libraries and community centers. The result is a year-round continuum of classroom and out-of-classroom learning.
http://www.jason.org/public/whatis/start.aspx

Exploratorium:
With over 12,000 web pages exploring hundreds of different topics, the Exploratorium is a collage of over 650 science, art, and human perception exhibits.
http://www.exploratorium.edu/index.html

The Tech Museum of Innovation:
A hands-on technology museum devoted to inspiring the innovator in everyone.
http://www.thetech.org/

The National Inventors Hall of Fame:
Celebrate the creative and entrepreneurial spirit of great inventors showcased on this web site through exhibits and presentations.
http://www.invent.org/index.asp

NASA:
Enter the NASA portal to explore the many facets of NASA. Learn the latest in space news; find math, science, and technology activities for home or the classroom; shoot a cannonball into orbit; or design your own mission to Mars. It’s all just a few clicks away.
http://www.nasa.gov

Kids Design Network:
Investigate a challenge, dream up a design, and draw your plans on the computer. Then, using the Internet, you can show your design to a real engineer! Best of all it’s free!
http://www.dupagechildrensmuseum.org/kdn/index.html

Society of Women Engineers (SWE):
An organization representing women in engineering and technical fields.
http://www.societyofwomenengineers.org

What Engineers Do:
Discover what engineers do, what the job is like, and even how much they are paid.
http://www.bls.gov/k12/text/mat_001t.htm

Discover Engineering™:
As you dig into this site, you will discover a wealth of information on engineering and careers. There are also some really "cool stuff" and games.
http://www.discoverengineering.org/

The National Inventors Hall of Fame:
Celebrate the creative and entrepreneurial spirit of great inventors showcased on this web site through exhibits and presentations.
http://www.invent.org/index.asp

NASA:
Enter the NASA portal to explore the many facets of NASA. Learn the latest in space news; find math, science, and technology activities for home or the classroom; shoot a cannonball into orbit; or design your own mission to Mars. It’s all just a few clicks away.
http://www.nasa.gov

Association for Women in Mathematics (AWM):
A nonprofit organization dedicated to encouraging women and girls in the mathematical sciences. AWM also offers a mentoring program.
http://www.awm-math.org