All the Best Linux Cheat Sheets
Linux Reference Card - Great reference published on FOSSwire website
One page Linux Manual - Great one page reference to the most popular Linux commands
Unix Tool Box - An incredibly exhaustive reference for all things Linux.
Treebeard's Unix Cheat Sheet - A great reference with Dos comparisons
Terminal Shortcuts - Cheat sheet for the most common terminal shortcuts
More Terminal Shortcuts - More shortcuts for history and X
2. Linux Security
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
netcat - Reference to the swiss army knife of networking
3. Linux Administration
Linux Admin Quick Reference - PDF cheat sheet of common Linux administration tasks
Crontab Reference - Also see Learning Cron by Example
4. Gnome/KDE
KDE Cheat Sheet - KDE specific cheat sheet
5. sed/awk/vim and other tools
Vi Cheat Sheet - Who doesn't love Vi/Vim. Also check out my guest post on Vim.
awk nawk and gawk cheat sheet - Awesome awk sheet, also check out awk is a beautiful tool
Screen Quick Reference - Quick reference to a must have screen emulator, Also check out Introduction to GNU Screen
Screen Terminal Emulator Cheat Sheet - PDF cheat sheet for screen
Vi/Vim Cheat Sheet - Nice vim/vi keyboard layout reference
6. Distro Specific
Ubuntu Cheat Sheet - Ubuntu specific cheat sheet
Debian Cheat Sheet - Debian specific cheat sheet
7. Everything Else
- Online Man Pages - The ultimate cheat sheet repository. Also use the 'man' command.