My tutoring resources folder containing real SAT / ACT / PSAT tests and book PDFs (email me)
College Panda SAT / ACT advice (dozens of good articles)
College Panda TI-84 SAT Calculator Program and Workbook (solves systems of two equations, quadratics, circles, lines, and much more). Comes with a purchase of any two new paperback College Panda books on Amazon.
SuperMemo Wiki on Learning (the best resource on learning I've seen)
Reading recommendations for high schoolers:
Standard reading recommendations to improve your SAT / ACT verbal scores
Everything on Wikipedia
Brian Tomasik's essays: briantomasik.com/writings
SPARC Book Recommendations. But really, just read anything!