Ages 12+ years, with some content for younger learners
Free; paid accounts starting at $19.99 per month with other plans available for semester, year, and lifetime

Brainscape is an adaptive flash card app and site that comes preloaded with excellent study sets for students in high school and beyond. For example, there are flash card sets to study for the driver's ed exam in several states, decks for AP classes, and so on. Professionals can find study sets for passing standardized vocational exams, too. With a paid account, you can make custom study sets. There's a little content for younger students, such as sight reading cards, but Brainscape is most suitable for learners 12 years and up.

Ages 13+ years
Free

CoolMath is a free site that explains pre-algebra, algebra, and pre-calculus concepts in ways young students might not have come across. By getting a fresh take on, say, polynomials, students ages 13 and up have a great chance at getting many mathematical concepts to click.

Ages 12 years and younger
Free

The same group behind CoolMath makes CoolMath4kids, which is suitable for younger kids working on more basic math concepts. This site covers addition, subtraction, multiplication, division, and fractions. It's more interactive than the version for older learners, with brain teasers, games, and quizzes.

Ages 7–17
Free

Encourage kids ages 7 to 17 to read age-appropriate news on DOGOnews. This site has timely articles on civics, current events, the environment, science, the world, and other topics that use language that young people can understand. It's available in English and Spanish.

Ages 5+
Free

Khan Academy is a virtual classroom for scholastics: computing, economics, history, language arts, life skills, math, science, and reading. For years, it has helped students of all ages learn everything from personal finance to phonics. You learn primarily by watching videos, but there are quizzes, too. Kids ages seven and younger who might have a hard time navigating the website will find lessons and games that are appropriate for them in an easy-to-use mobile app called Khan Academy Kids.

Ages 5–17
Free

The Monterey Bay Aquarium has a subsection of its site called Learning at Home with online courses and activities for young people to learn about ecosystems, marine animals, ocean conservation, and related topics. The site is available in English and Spanish. In addition to online courses, which you can view by grade level, the site also has crafts, short readings (facts and photos) about various animals and plants, and suggestions for parent-led activities.

Ages 5–16
Free

The online kids' version of National Geographic gives young people educational articles, quizzes, and videos on a variety of topics, including animals, history, science, and space. National Geographic Kids isn't as interactive as some sites, but it's good for short articles on timely topics, such as a kids-appropriate history of Juneteenth. With a name like National Geographic, however, it is very US-centric in what it covers and how.

Ages 5–17
Free

PBSLearningMedia has a wealth of content that teachers can use or that students who are old enough to use a computer unsupervised can independently explore. It has videos, as well as interactive lessons that teach art, engineering, health and physical education, math, science, social studies, and other subjects. For very young children (ages 2 to 5), you can find more age-appropriate shows and interactive content at PBS Kids; we didn't include it officially in this list because it's a bit of a stretch to call it a learning site, though some content may be educational.

Ages 8+
Free limited version; paid plans $7.99 per month or $35.99 per year

Quizlet started as a flashcard app but now includes games, quizzes, and other ways for young people to learn, review, and study. With Quizlet, you can create custom decks of material to study or look for content that other people uploaded and shared. A free version gives you limited ways to interact with your content, while a paid account unlocks almost everything else. Some study decks shared from professional organizations cost extra.

Ages 5–11
Free

The Scholastic Kids website is a great resource of educational reading material for kids approximately 10 years and younger. Here, you can find short educational articles for kids who are curious about animals, genetics, geography, roller coasters, genetics, and other topics. You can sort by grade level to find articles that are appropriate for the young learners in your life.

Ages 2–5
Free

For the littlest learners, nothing beats the classic educational content from Sesame Street or Sesamo (if you want the learning experience in Spanish or Portuguese). The two-to-five-year-old crowd can use this interactive website to make art, play games, watch videos, and more. The educational aspect is mostly in the form of learning how to develop motor skills, follow directions, identify basic shapes (like animals), and so forth.

Ages 5–9
$35 per year for a Home membership; some content is free

Starfall mostly focuses on content that helps children learn and practice simple math. It's reasonably engaging, too. Some content is available for free, but you need a $35-per-year membership to access everything on the site. It’s only $70 per year to give an entire elementary school access, so ask your school administrator.

Young adults and mature children
Free

TED-Ed is TED's youth and education initiative. It's a site where you can find short educational videos about a wide range of topics, including those related to current affairs. While the site doesn't seem to have an official recommendation regarding the appropriate age group for its content, one video we previewed referenced HIV/AIDS and condoms, so it's perhaps most suitable for young adults and mature children who are capable of either understanding or asking questions about some advanced topics.

Ages 6–11
Free

TIME Magazine has an online learning site for kids with entertaining and educational articles (available in English and Spanish). TIME for Kids articles have interactive elements, such as a word look-up feature that helps young learners expand their vocabularies. You can find book reviews for kids by kids, as well as insights on conservationism, health, sports, and more. You can also sort articles into age categories to make sure your young learners get articles that are both interesting and accessible to them.