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

This is a linux command line reference for common operations (HTML format).
Linux Reference Card published on FOSSwire website by Jacob. (PDF format)
A summary of useful Linux command by Squadron. (PDF format)
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 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.Ubuntu Cheat Sheet
There comes a time when one needs an end-all reference to the system. The time is now, and if you’re an Ubuntu user you’ll like this cheat sheet. (PDF format)
7.VIM Graphical Cheat Sheet
Probably the best Vim 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)
Read more >> (Update: the link is down. I’m sorry)