desmos.com
Desmos offers best-in-class calculators, digital math activities, and curriculum to help every student love math and love learning math.
gutenberg.org
Project Gutenberg is an online library of free eBooks.
thistothat.com
Check whether you can glue certain things to other things. Recommends a glue.
sci-hub.se
Sci-Hub is the most controversial project in modern science. The goal of Sci-Hub is to provide free and unrestricted access to all scientific knowledge.