Linux and Unix

1.Linux Command Line Tips
2.Unix/Linux Reference Card
3.One Page LInux Manual
4.Linux Security Quick Reference
5.Screen VT100/ANSI Terminal Emulator Cheat Sheet
6.Vi/VIM Graphical Cheat-Sheet

Ubuntu unleashed is a great site for Ubuntu users. It has Ubunutu Tweaks, Themes, Apps, CLI, Tricks, Hacking, Cracking and Customization .

A good collection of articles on above topics.

HowtoForge provides user-friendly Linux tutorials about almost every topic.

A large collection of eBooks teaching users everything from the basics to advanced linux.

Carry a portable Linux version with you on a USB flash pen drive. Bring your portable desktop with you wherever you go.

Techotopia is a library of free on-line IT books covering a wide range of topics including operating systems, programming, scripting, system administration, databases, networking and much more.

Free Online book on Ubuntu Linux Essentials

Free online Linux network administrator guide

Tutorialstuff.com for all the latest tutorials in Operating Systems, Programming, Software,Browser,etc.

CertCities.com Linux/Unix News an online magazine for the certified IT professional community. on what is happening in the Linux/Unix  field

Linux Study Guide contains :

Editor's Letter
Overview of Linux Certification Programs
Preparing for the RHCE Exam
How to Study for the Linux Exams
An Overview of Vendor-Neutral Linux Study Guides

Linux HOWTOs are documents which describe in detaila certain aspect of configuring or using Linux.

This site covers topics needed for Linux software certification exams, such as the RHCE, and many computer training courses.

A self-study workshop to review and/or learn a wide range of Unix tools, including shell scripts, awk, lex, yacc, grep etc.

This course teaches students how to configure server serivices such as the Apache web server, the Squid Proxy server, the Bind name server ,Sendmail and Exim Mail servers are discussed.

This course teaches students how to create shell scripts, the use of Regular expressions, file manipulation with sed, grep and awk and how to tie all this new knowledge into scripts that make using Linux so simple.

This course is designed to help the student understand the inner workings of the Linux kernel.

This course includes the history of Linux and what distinguishes Open Source Software from proprietary software. On the practical side students learn the fundamental commands and approach needed to start using Linux.

Part 1 Choosing Linux
Chapter 1. Introduction
Chapter 2. The case for migration

Part 2 Planning the pilot migration
Chapter 3. Organizational and human factors planning
Chapter 4. Technical planning

This book provides a set of design and implementation guidelines for writing secure programs for Linux and Unix systems.