What are the best study guide sites for reviewing school subjects before exams?

Here are some of the best study guide sites for reviewing school subjects before exams, grouped by what they’re best for:

General study help

• Khan Academy — Great for math, science, history, and some test prep. Clear lessons and practice questions.
• CK-12 — Good free textbooks, summaries, and practice across many school subjects.
• SparkNotes — Best for literature, books, and some history topics. Easy-to-read summaries.

Math and science

• Khan Academy — Excellent for step-by-step practice.
• Paul’s Online Math Notes — Very helpful for algebra, calculus, and higher-level math.
• Crash Course — Good quick video reviews for biology, chemistry, physics, and history.
• Science Olympiad/Wiki resources — Useful for deeper topic review if you need extra practice.

Flashcards and memorization

• Quizlet — Great for vocab, definitions, formulas, and quick self-testing.
• Brainscape — Similar to Quizlet, with spaced repetition for memory work.

Test prep and structured review

• College Board — Best for SAT/AP prep resources.
• ACT.org — Official ACT practice and explanations.
• CK-12 — Useful for school-level subject review and practice.

Writing, English, and literature

• Purdue OWL — Excellent for grammar, essay writing, and citations.
• NoFear Shakespeare — Helpful for understanding Shakespeare plays.
• SparkNotes — Strong for summaries, themes, and character analysis.

Quick video explanations

• Crash Course
• Khan Academy
• The Organic Chemistry Tutor — Especially good for math and science problem-solving.

Best overall picks

If you want just a few:

1. Khan Academy for most subjects
2. Quizlet for memorization
3. SparkNotes for English/literature
4. Purdue OWL for writing
5. Crash Course for fast review

If you want, I can also give you:

• the best sites by subject,
• free vs paid options, or
• a high school / middle school / college version.