Editors
emacs Reference Card (more intuitive)
vi Reference Card (available on all Linux systems)
UNIX / Linux
basics
Interactive tutorial
Tutorial for beginners
UNIX Reference Card
utilities
Shells (bash, ksh)
Some shell basics
bash Beginners Guide
ksh scripting examples
Manipulating strings with bash
Most used commands
Other programming
Fortran
Programmer's Guide to Fortran77
Parallel programming / High Performance Computing (HPC)
Introduction to parallel programming
Introduction to parallel computing
A parallel computing tutorial