GCC:

• An introduction to GNU attributes
• An Introduction to GCC for the GNU Compilers gcc and g++ good explanation on compilation options and etc
• GCov for code coverage
• Valgrind for memory check
• G Profile
• Automake
• A list of discussions on static compilation:
  • G Plus Plus Minus One
  • Road to Hells Is Paved with Good Intentions
  • How to link libstdc++ statically

Make:

• Make Menual
• Recursive Make Considered Harmful A paper on recursive Make and suggestions on compiling multil-directory source files

XMLRPC

• Specification
• C/C++ library
• Java Library

Perl:

• Essential Perl A quick introduction to Perl. Very good for beginners.

Python:

• Python Tutorial
• Networking in Python (a tutorial slide)
• Library Reference Very concise description (e.g. in regular expression

Awk:

• An Awk Tutorial Quick overview and clear syntax.

Shell scripts:

• For Mac clear explanation of different between .bashrc and .bash_profile
• Steve's Bourne / Bash shell scripting tutorial I think this book is more careful in terms of the usage of different names and etc. Should follow them to avoid lots of small mistakes
• Linux Shell Scripting Tutorial -- A Beginner's handbook I like this book. It covers topics that are most useful in shell. An other location for this book.
• Shell Script Programming A short discription on BASH programming -- in one page, which means it is easy to search.
• Linux Programming A simple tutorial for programming in Linux: including shell, c/c++ and X

GNU Plot:

• GNUPLOT 4.0 - A Brief Manual and Tutorial A quick tutorial for beginner.
• Introduction to GnuPlot More details on parameters such as line sytle and etc.
  
  
• Master Manual
  

SQL:

• SQL Tutorial If you are looking for syntax of SQL... very easy to navigate and find your answer.
• MySQL reference manual For lookup. From installation, account management to sql syntax and performance. very detailed.
  

PHP:

• 3 Days to learn PHP/MySQL (in Chinese) Very simple tutorial for beginners to learn how to install and program php with mysql. Be careful that some of the commands are out-dated or non-recommended and you'd better to use it with MySQL reference manual and PHP manual if you want to master the techniques accurately.
  

• PHP manual For lookup. again, every details.
  

HTML:

• HTML Tutorial

• ClearSilver

Tcl/Tk:

• Tcl/Tk information by Team Niehaus There is a simple tutorial which is good for beginners. Also, there is an online book (Tcl and the Tk Toolkit by John Ousterhout). They also have links to other resources.

OTCL:

• Information from Berkeley Continuous Media Toolkit Project A simple tutorial for OTCL. API descriptions. Good examples.

GDB:

• GDB tutorial. Very simple one-page tutorial of GDB.

GProfile:

• GProfile Tutorial Very simple one-page tutorial of GProf.

Latex:

• The Not-So-Short introduction to Latex "Latex in 139 minutes". I like it as a handbook for Latex.
• Latex Wiki

NS2:

• NS by Example short and simple tutorial for beginners.
• NS Manual The thick book with all details. Make sure to check all the parameters with it before you run a new ns simulations.
• A NS-2 Tutorial There are some good examples for data recording (using monitor instead of trace)

Sendmail:

• a concise tutorial for simple mail service configuration, from very beginning concepts

Thread programming:

• Linux How-to
• POSIX Threads Programming

• Maximum RPM It is very clear in how to use and how to build an RPM.

Unix Administration topics:

• UUALA

Security

• SANS Institute many good white papers

Matlab

• Using MATLAB for presentation Many good tricks in making matlab figure better