Programming (physics):

• http://www.nrbook.com/a/ - The complete "Numerical Recipes" books in C, Fortran 77, and Fortran 90 On-Line, in Adobe Acrobat format.
• http://www.mathworks.com/matlabcentral/fileexchange/ - The file exchange server is an invaluable resource, when programming in MATLAB.
• http://www.netlib.org/lapack/lug/ - The LAPACK users' guide (Third Edition).

Molecular structure:

• http://webbook.nist.gov/chemistry/ - The NIST chemistry WebBook provides thermochemical, thermophysical, and ion energetics data compiled by NIST under the Standard Reference Data Program.
• http://www.msg.ameslab.gov/GAMESS/documentation.html - From the Gordon Group/GAMESS homepage, you can download the molecular structure calculation software package GAMESS free of charge.
• http://gabedit.sourceforge.net/ - Home page for Gabedit. Gabedit is a graphical user interface to computational chemistry packages like Gamess-US, Gaussian, Molcas, Molpro, MPQC, OpenMopac, PCGamess and Q-Chem.
• https://bse.pnl.gov/bse/portal - The Basis Set Exchange has a database with basis sets that can be used with GAMESS-US by following the recipe on Damian Gregory Allis's home page.

Plotting:

• http://t16web.lanl.gov/Kawano/gnuplot/index-e.html - A detailed and extremely useful manual for GNUPLOT.

Scientific English:

• http://www.natcorp.ox.ac.uk/ - The British National Corpus is a brilliant database with English texts, which allows you to check phrases and grammar.
• http://users.wpi.edu/~nab/sci_eng/ - A nice guide when writing scientific reports or articles in English.