Resources
Physics and Math
- MathWorld - An excellent resource for looking up math concepts.
 - Eric Weisstein’s World of Physics - Similar to MathWorld, only with physics.
 - HyperPhysics - Another great (though more elementary) physics resource.
 - Maxima - An excellent GPL’d computer algebra system. I’d recommend SymPy instead these days, however.
 - Falstad Math and Physics Applets - Some excellent applets on math, quantum mechanics, electrodynamics, etc.
 - Electromagnetic Waves and Antennas - A freely available book (with inidividual chapters in PDF form).
 - Encyclopedia of Laser Physics and Technology
 
Python
reStructuredText
I previously used reST for notes and as a plaintext format. I’ve now switched over mostly to Markdown, but since reST is used heavily for Pythonic documentation, it’s still good to know.
LaTeX
- Getting to grips with Latex - An excellent introduction to LaTeX.
 - LaTeX Beamer - A LaTeX style class for creating presentations. Check out the themes gallery.
 - LaTeX Letter Format - For those times when you need to actually send a letter, this is how you do it with LaTeX.
 
Linux
- Debian - The best Linux distribution out there.
 - autotut - A guide to using the GNU autotools (autoconf, automake, autoheader).
 - Gnuplot Not So Frequently Asked Questions - An incredible overview of gnuplot.
 - Advanced Bash-Scripting Guide - Everything you ever needed to know about Bash scripting.
 - Filesystem Hierarchy Standard - Information what everything in / on your Linux system actually means.
 
Electronics
- Dangerous Prototypes - Many interesting open source hardware projects
 - dirtypcbs.com - Cheap PCB service (China)
 - Seeed Studio - Electronics components and cheap PCB fabrication (China)
 
KiCAD
HOWTOs
Arduino
- Ethernet