Math

Online Encyclopedia of Integer Sequences (OEIS): A classic. A lifesaver in solving combinatorics or programming problems by any means necessary, as you can just plop any numbers you find into this thing and hope something nice comes up. (Seriously, if you do Codeforces or Project Euler, THIS IS ALLOWED, please do it!) If you're not there to solve problems, I reccomend searching up your favorite sequences and hitting the "listen" button to listen to them as MIDI files.

RILEYBOT Inverse Equation Solver (REIS): Math books written in the 80s and 90s love hyping up MATLAB. In the 21st century, books should be hyping up this, as well. You're probably familiar with the age-old tactic of numerically estimating a solution to something, then checking it with any answer received. This site can take the numerical approximation and return a likely form for the actual answer. For example, if "8.372211626601275661625747121" is entered, it will give 4arccot(sqrt(phi)) as an answer (the value of the famed Cleo integral!)

Encyclopedia of Triangle Centers: Did you know a triangle has over 80 thousand centers? (I suppose you shouldn't be *THAT* surprised; any definition can be stretched arbitrarily far.) Also check out any of the sites that this site links to, and see the applications to polynomial curves. Anyone who doesn't think this is cool is wrong.

The L-Functions and Modular Forms Database: A classic. Whenever I think about category theory too much, I visit this site to remember how much better number theory is. This site lies at the cross section of so much beautiful math.

The n-Category Cafe: It's a nice blog. I can't comment on n-categories.

Programming

Project Euler: THE resource for great programming problems. These range from pretty routine exercises to what are practically mini-research projects. These problems are more math-focused than databases like Leetcode or CSES (which makes it better, obviously). I also reccomend joining the Project Euler discord server to discuss problems and programming.

Utility

Detexify: You're writing something in LaTeX, and you have a problem. There's a symbol you have to type, and you know what it looks like, but not how to type it. That's fine, just google "[symbolname] in latex." But what if you don't know the name of the symbol either? Open up Detexify and DRAW the symbol in the box, and it will tell you how to type it.

Misc

Crab Database: It's a beautiful world out there. Look at some beautiful crabs.

An Orange Cat For 200 Million Dollars: Read my favorite ever comment to ever grace the internet.