LaTeX

Inessential LaTeX (Revision: 3.39)
Other on-line LaTeX information
Hypertext Help with LaTeX
Help On LaTeX Commands
Help On LaTeX Math Symbols
LaTeX Symbols

Vim

Vim and Vim help

C

Eddie's basic guide to C

Python

Learn Python the hard way
Python documentation/library

Other

Sed intro and tutorial
PGPLOT from Caltech
Grace plotting package
Concatenate .ps files into .ps or .pdf (click on Postscript and PDF)
xine
TinyURL

Amir Karger's Scriptome project of useful Perl scripts