LaTeX is a high-quality typesetting system; it includes features designed for the production of technical and scientific documentation, including mathematics!

"Overleaf is a free service that lets you create, edit and share your scientific ideas easily online using LaTeX, a comprehensive and powerful tool for scientific writing."

A free resource from Wolfram Research based on Eric Weissteinn CRC Concise Encyclopedia of Mathematics.

MathDL is an online resource published by the Mathematical Association of America. The site provides online resources for both teachers and students of mathematics. Go here ==> https://www.math.uni-bielefeld.de/~rehmann/DML/dml_links_author.html for complete list!

A free electronic copy of NIST Handbook of Mathematical Functions. Contains notation, properties, applications, and computation.

National Museum of Mathematics hosts this online event every Friday. Come in and practice the mathematics of origami.

Actually, the National Museum of Mathematics has many great online events happening. Check out here to see their full schedule.

The Carnival of Mathematics is a monthly blogging round up hosted by a different blog each month, featuring any mathematics-related blog posts, YouTube videos or other online content.

A monthly selection of mathematical news and stories.

Mission: "To advance mathematical knowledge through collaboration, to broaden participation in the mathematical endeavor, and to increase the awareness of the contributions of the mathematical sciences to society."

Associated with UC Berkeley, they are dedicated, "to the advancement and communication of fundamental knowledge in mathematics and the mathematical sciences."

MoMath, the National Museum of Mathematics, is an award-winning museum that highlights the role of mathematics in illuminating the patterns and structures all around us.

"The website for LGBTQ+ mathematicians and their allies. This arose from a need for recognition and community for Gender and Sexual Minority mathematicians, and we hope that this will be a resource for our community."