This is currently only a very limited index of the things I enjoy on the internet. I'll do my best to update it every so often with new Cool Things.

Software

CadQuery: OpenSCAD, but designed in the past century: python, constraints, references, STEP export, and more.
Ondsel: FreeCAD, but usable
Zip-o-mat's Slic3r: Nonplanar slicing!
WORHP: decent NLP solver (SQP method) whose name stands for We Optimize Really Huge Problems (helped me fix my life)
ForcesPRO: create fast embedded optimizers, for free!
fuckit.py: the greatest python library to ever exist. "This module is like violence: if it doesn't work, you just need more of it."
qlock, a js quine clock

Hardware

RepRap: the wonderful project that brought low-cost rapid prototyping to everyone
Odrive: a beautiful, open-source brushless motor driver
Rack Robotics Powercore: Wire EDM for everyone
Rotoforge: an innovative, cheap approach to metal 3D printing
Poma Lifts, so many bandaid solutions piled together that it's an engineering marvel

Shigley's Mechanical Engineering Design, one of the hardware bibles
Principles of Robot Motion: Theory, Algorithms, and Implementations, also known as the software bible or the blue book
media.catboy.camp, a repository of higher level math, physics, and robotics texts, research papers, and delicious recipies

mkremins' gender quiz: solve your identity crisis today. The last gender quiz you'll ever need.
Kuriosity Hardware Wiki, a snapshot of the documentation I wrote for Kuriosity hardware. Not actively maintained.
AstroAlpaca, a fun game