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
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.
- Getting to grips with Latex - An excellent introduction to LaTeX.
- Online Tutorials for LaTeX - Some more LaTeX tutorials.
- 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.
- James Bray's Latex Help - Various useful tips with selected topics in LaTeX.
- 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.
- Dangerous Prototypes - Many interesting open source hardware projects
- dirtypcbs.com - Cheap PCB service (China)
- Seeed Studio - Electronics components and cheap PCB fabrication (China)