Richard Feynman lectures on QED, a life-time inspiration.
How to become a good theoretical physicist, by Nobel prize laureate Gerard 't Hooft.
Homepage of Prof. Eduardo Fradkin at UIUC. Very good resources about field theory and condensed matter physics can be found in his course webpages.
Advanced Quantum Mechanics course page by Prof. Judith McGovern in Manchester University.
Homepage of Prof. Andrei Postnikov, where I found the best materials about Density Functional Theory and electronic structure calculation.
Technique and Applications of Quantum Monte Carlo, PhD thesis of Paul Kent at Cambridge University, good review of electronic structure calculation and QMC.
Graduate Quantum Mechanics Notes, by Prof. Michael Fowler at University of Virginia.
Adv. Topics in Superconductivity course webpage, UC Davis
Advanced Classical Electromagnetism, a graduate level course of lectures given by Prof. Richard Fitzpatrick at the University of Texas at Austin.
Semiconductor Electronic Structue & Optical Processes, lecture notes by Prof. Ceyhun Bulutay at Bilkent University.
Math Pages, an anonymous website with a collection of interesting stuff on mathmatics.
Geometry and Topology in many-particle physics, website of a course given by Prof. Joel Moore at UC Berkeley.
Naval Research Lab Crystal Lattice-Structures Database
Springer Materials The Landolt-Börnstein Database
NIST Atomic Reference Data for Electronic Structure Calculations
DFT Codes and Websites, collection of DFT resources and documents, by Sandia National Lab.
NIST Digital Library of Mathematical Functions
Prog Archives, progressive rock collections.
Dream Theater, my favorite.
Website about David Gilmour (Pink Floyd)'s Gears, where I found useful information about practical techniques such as maintenance of your instruments.
Tom Gootee's Guide of Home-Made PCBs
Kenneth Maxon's Tutorial on How to Solder Surface-Mount Components Using an Oven
How to become a hacker, by Eric Steven Raymond.
Tex Resources at UIUC math, a good collection of online resources on LaTex.
Beginning Perl, free online book on Perl.
The Command Line in Windows, about the Windows command line, batch files and scripting
WebPlotDigitizer, a useful webtool to extract data from published figures.