Need a quick reference card? Here you have a list you can choose from:

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
7.Firefox Keyboard and Mouse Shortcuts
8.Gimp Reference Card
9.Debian Reference Card
10.Google Cheat Sheet

Read more here: 10 must-have Linux (and not only) cheat-sheets