Every (ethical) hacker has one wall in his computer lair where he pins all his paper sheets with useful information. This varies from common IP ports, from common linux commands to in depth VI commands. What cheat sheets do you have printed out and pinned on the wall behind your computer? (i dont have any cause mine is in our livingroom, my girlfriend would kill me!)

i use:
common IP ports
common rootkit ports
VI commands
linux commands

Here are some good links:

http://www.cheat-sheets.org/
http://whatis.techtarget.com/definition/0,,sid9_gci826135,00.html
http://packetlife.net/library/cheat-sheets/

Here's a couple I use:

http://www.4shared.com/file/253608993/d7ec86e1/Cheat_Sheets.html

Inside contains a misc tool cheat sheat (covers basics of hping, fping, fgdump & some metasploit), a netcat cheat sheet (this has proved very useful to me!), a windows command line cheat sheet & also a Google Hacking & Defense Cheat Sheet. Enjoy.

http://www.packetstormsecurity.org/papers/cheatsheets/

good onces here.

http://img530.imageshack.us/img530/6519/gmailhardening.png
(gmail hardening cheat sheet)

I know it's an old topic. But I found some new cheat sheets and didn't want to make a new thread.
Approx 70 cheat sheets for Linux users.
http://www.scottklarr.com/topic/115/linux-unix-cheat-sheets---the-ultimate-collection/

Good collection here: devcheatsheet.com