1.Linux Command Line Tips

This is a linux command line reference for common operations (HTML format).

2.Unix/Linux Reference Card

Linux Reference Card published on  FOSSwire website by Jacob. (PDF format)

3.One Page LInux Manual

A summary of useful Linux command by Squadron. (PDF format)

4.Linux Security Quick Reference

The intent of this Quick Reference Guide is to provide a starting
point for improving the security of your system, to serve as a pointer
to more in-depth security information, and to increase security
awareness and methods that can be used to improve security. (PDF format)

5.Screen VT100/ANSI Terminal Emulator Cheat Sheet

This cheat sheet summarizes all they default keyboard mappings, with
screen’s commands to execute the mapping and a description of each
mapping. (PDF format)

6.Vi/VIM Graphical Cheat-Sheet

This is a single page describing the full vi/vim input model, the
function of all keys, and all major features. You can see it as a
compressed vi/vim manual. (GIF format)

7.Firefox Keyboard and Mouse Shortcuts

Here is a list of Firefox keyboard and mouse shortcuts for each of
the three major operating systems organized by topic.(HTML format)

8.Gimp Reference Card

Designed for Gimp version 1.2.2. (PDF format)

9.Debian Reference Card

The 101 most important things when using Debian GNU/Linux (last revision 2005-09-03). (PDF format)

10.Google Cheat Sheet

This two page Google Cheat Sheet lists all Google
services and tools as well as background information. The Cheat Sheet
offers a great reference to grasp of basic to advance Google query
building concepts and ideas. (PDF format)

You can find more useful cheat-sheets on Peteris  great site:  www.catonmat.net