Build your own (insert technology here) when you know nothing about it. Dozens of links to curated tutorials about lots of different things, from 3D rendering to how network stacks work to other stuff.
How to train yourself to dream lucidly.
A wiki dedicated to the art of contact juggling.
A wiki of designs, schematics, and links to open source electronics projects. If you get lost in here long enough you'll find some interesting stuff.
A Creative Commons-licensed book by Al Sweigart that teaches programming in Python through writing your own video games. Full source code is included.
A wiki that documents many tips and tricks for writing shell scripts in Bash.
A site that has presents a step-by-step howto for getting involved with OpenStreetmap.
A wiki of resources for people writing bots - actual bots to interact with, tools, tutorials, code, and datasets. exocortex chatbots howto twitter
A wiki of ways to fix things, including teardowns, where, to get parts, video tutorials, and diagnostic procedures.