General

Khan Academy: many videos and articles. Also has computer programming basics with helpful community discussions and support. (my profile)

Math

Notes and study materials.

Trig Functions flashcards

Volume flashcards

Circles and Parabolas

Science

Dynamic Periodic Table

Metric System Chart

Open-Access Chemistry Info (plus biology, statistics, and more)

English

Punctuation Guide: the important thing in life is to know where to stick your quotation marks, the American way. The British rules make more sense but ¯\_(ツ)_/¯ Alt+0151 makes an em dash.