Here are some links to cool math stuff that I think more people should see.
Bathsheba Sculpture: A variety of beautiful sculptures inspired by math and nature. I bought my car's keychain here.
Brilliant: An educational service that provides a large number of math and science courses. It promotes a model for learning that focuses on self-paced, interactive lessons taught through puzzles, an approach which I hope becomes more prevalent as technology becomes more accessible.
The Center for Election Science: This is a great educational resource for learning more about voting theory, which in turn can help you to understand why the current political climate is so bad. In particular I would recommend you check out their about their articles about how first-past-the-post is a terrible voting system, and how approval voting is much better.
Distill: An online journal for machine learning. All of their articles contain ample visualizations, many of which are even interactive. I would also highly recommend their article on research debt, which emphasizes the importance of researchers being able to explain their work in a way that is easy to understand.
Math Pickle: A math education website that promotes teaching through use of puzzles and games, which I am all for. It includes a large library of puzzles for K-12 students. Most of the puzzles are highly visual and rely on inductive reasoning, and many of them provide a great way to introduce students to major open problems in mathematics.
OEIS: The Online Encyclopedia of Integer Sequences is an invaluable resource for any research mathematician. Anybody whose work produces a sequence of integers usually searches for it on the OEIS first to see whether it has been studied before and whether it has any known properties. It also has some fun built-in tools for visualizing and audiating interesting sequences (like the forest fire sequence).
Punk Rock Operations Research: The blog of Dr. Laura Albert, the person whom originally got me interested in operations research.
Quantum Minigolf: I first discovered this game when I was in high school, and apparently it left quite the impression because I still think about it. It is the first example I saw of a game being used to teach a normally abstract and alien concept (in this case quantum mechanics) by placing the player in a world that operates according to a different set of rules. I think that this is one of the great potential strengths of games as an interactive teaching medium, and in many ways it formed the basis for my own game projects.
The Riddler: A weekly puzzle column from FiveThirtyEight. These puzzles are quite challenging and often require advanced mathematics, and many of them are open-ended enough to encourage a wide variety of creative approaches.
Science vs Magic: A collection of visually-striking browser-based games that cover a variety of topics, including geometry, tessellations, fractals, and cellular automata. The visual design was clearly inspired by Escher, which is always a good thing.
Toroidal Snark: The home page of Dr. Sarah-Marie Belcastro. Interesting mathematical knitting projects and some very helpful guides to math university programs and math writing. I would recommend that anyone considering going into a graduate math program check out this site.
Topology and Geometry Software: A collection of digital toys and games that illustrate abstract mathematical concepts, like non-Euclidean space and high-dimensional space.