Khan Academy
Topics: Counting, addition/subtraction, fractions, decimals, pre-algebra, basic geometry, intro science, grammar
Prerequisites: None
Khan Academy
Topics: Algebra I & II, Geometry, Trigonometry, Precalculus, Calculus AB/BC, Biology, Chemistry, Physics, AP exam prep, SAT prep
Prerequisites: Pre-algebra mastery; Precalculus/Trig for Calculus
Khan Academy
Topics: College Statistics, Personal Finance, Computer Programming (JS/SQL/HTML/CSS), remedial math for career changers
Prerequisites: Algebra I for Statistics; varies by module otherwise
Coursera
Topics: Python for Everybody, Intro to Data Science, Google IT Support/UX/Data Analytics Certificates
Prerequisites: Basic computer literacy; HS-level math for data tracks
Coursera
Topics: Machine Learning Specialization, MBA-style business courses, full online Master's degrees, Deep Learning, Cloud certifications
Prerequisites: Linear algebra, calculus, Python (ML); bachelor's degree (Master's/MBA tracks)
edX — CS50 & Intro Courses
Topics: CS50 (Intro to Computer Science), Intro Statistics, Intro Biology
Prerequisites: None — CS50 explicitly starts at zero
edX — MicroBachelors/MicroMasters
Topics: MicroBachelors programs, MicroMasters (Supply Chain, Data Science, Cybersecurity)
Prerequisites: HS diploma-level knowledge
edX — Executive & Professional
Topics: Executive Education, MicroMasters-to-Master's pathways, AI/ML, blockchain, engineering certificates
Prerequisites: Relevant bachelor's degree typically expected
MIT OpenCourseWare
Topics: Intro to CS with Python (6.0001), Single Variable Calculus (18.01), Classical Mechanics (8.01)
Prerequisites: HS-level algebra/trig for calculus and physics
MIT OpenCourseWare — Advanced
Topics: Algorithms (6.006), Linear Algebra (18.06), Artificial Intelligence, graduate seminars
Prerequisites: Completed intro calculus/discrete math/programming sequence
Duolingo
Topics: Alphabet/pronunciation, basic vocabulary (colors, animals, family), simple present-tense sentences
Prerequisites: Literacy in interface language (typically English)
Duolingo
Topics: Grammar structures (past/future tense, conditionals), everyday conversation, listening comprehension
Prerequisites: Completion of basic vocabulary units
Duolingo
Topics: Idiomatic expressions, Duolingo English Test prep, advanced reading/listening, fluency practice
Prerequisites: Intermediate-level unit completion
Codecademy
Topics: HTML/CSS fundamentals, intro Python, intro JavaScript (variables, loops)
Prerequisites: Basic computer/keyboard literacy; no prior coding
Codecademy — Career Paths
Topics: Full-Stack Engineer Path, Data Science Path, React, SQL, APIs
Prerequisites: Completion of basics track (variables, functions, loops, conditionals)
Codecademy — Advanced
Topics: Machine Learning/AI, advanced Data Structures & Algorithms, Cybersecurity Path, DevOps
Prerequisites: Intermediate programming fluency; basic statistics/linear algebra for ML
Scratch (MIT Media Lab)
Topics: Drag-and-drop block sequencing, animations, sprite movement, loops, conditionals, variables, simple game logic
Prerequisites: Basic mouse/touchscreen use; reading skill for block labels
ABCmouse
Topics: Letter/number recognition, phonics, sight words, basic addition/subtraction, early reading fluency
Prerequisites: None
Brilliant
Topics: Logic puzzles, basic geometry, intro probability, Math Fundamentals
Prerequisites: Basic arithmetic
Brilliant
Topics: Algebra, Scientific Thinking, Intro Computer Science, Calculus
Prerequisites: Pre-algebra; logical reasoning skills
Brilliant — Advanced
Topics: Linear Algebra, Group Theory, Quantum Mechanics, Neural Networks, Differential Equations
Prerequisites: Calculus and linear algebra; programming basics for CS/ML-adjacent courses
Udemy
Topics: Marketplace covering coding, business, creative, and professional skills at every level
Prerequisites: Varies by individual course; listed on each course page
National Geographic Education
Topics: Animals/habitats, map basics, weather, climate science, GIS mapping, conservation
Prerequisites: None
Smithsonian Learning Lab
Topics: Primary-source collections, document analysis, art history, historical inquiry projects
Prerequisites: None
freeCodeCamp
Topics: Responsive Web Design (HTML/CSS/Flexbox/Grid), JavaScript Algorithms and Data Structures
Prerequisites: Basic computer literacy; no coding background
freeCodeCamp — Intermediate
Topics: Front-End Development Libraries (React), Back End (Node/Express), APIs and Microservices, Data Visualization
Prerequisites: Completion of JavaScript basics certification
freeCodeCamp — Advanced
Topics: Machine Learning with Python, Information Security, Data Analysis with Python, Relational Database certification
Prerequisites: Solid JavaScript or Python fundamentals; basic statistics for ML
Wolfram Alpha / MathWorld
Topics: Arithmetic checking, step-by-step algebra/calculus solving, unit conversions, advanced math reference (topology, number theory)
Prerequisites: Basic arithmetic (basic use) up to undergraduate-level math (MathWorld reference entries)
TED-Ed
Topics: Short animated explainer lessons, critical thinking, history, science, Student Talks for public speaking
Prerequisites: None
Skillshare
Topics: Beginner drawing, basic photography, intro creative writing
Prerequisites: None
Skillshare — Advanced
Topics: Advanced design systems, entrepreneurship/marketing strategy, advanced creative technique
Prerequisites: Prior intermediate-level skill in the craft area
PBS LearningMedia
Topics: Reading readiness, basic math, science exploration
Prerequisites: None
PBS LearningMedia — Standards-Aligned
Topics: History, biology, chemistry, physics, literature analysis lesson plans
Prerequisites: Grade-appropriate reading level
Harvard Online Learning
Topics: CS50, Intro to Data Science, Intro Statistics, professional certificates
Prerequisites: Strong HS-level math/reading; no prior coding for CS50
Harvard Online Learning — Executive
Topics: Executive Education, advanced data science/AI, graduate-level certificate programs
Prerequisites: Significant professional experience or graduate-level academic background
Stanford Online
Topics: Graduate certificates in Statistics, AI, Data Science (foundational courses)
Prerequisites: Bachelor's in a quantitative field typically expected
Stanford Online — Advanced
Topics: AI/ML graduate-level courses, executive/professional education in engineering and business
Prerequisites: Strong linear algebra, calculus, programming experience; professional experience for executive tracks
LeetCode
Topics: Data structures (trees, graphs, hash maps), Big-O complexity analysis, dynamic programming basics
Prerequisites: One semester+ of programming; Data Structures & Algorithms coursework or equivalent
LeetCode — Advanced
Topics: Advanced dynamic programming, graph algorithms, system design interview prep, competitive programming
Prerequisites: Solid CS fundamentals; typically used by working engineers