More Terminal Shortcuts - More shortcuts for history and X

Terminal Shortcuts - Cheat sheet for the most common terminal shortcuts

Unix Tool Box - An incredibly exhaustive reference for all things Linux.

One page Linux Manual - Great one page reference to the most popular Linux commands

Linux Security Quick Reference Guide - An awesome security checklist reference

IP Tables - If you are interested in Linux firewalls this is a must have

TCPDump - Great cheat sheet to an awesome security tool

Wireshark Filters - An awesome list of filters for the best packet sniffing utility

IP Access Lists - Cheat sheet for IP Access Lists

Common Ports - In case you don’t have all common ports memorized