The Magic SysRq Key Cheat Sheet
Every Linux kernel developer knows about the "Magic SysRq key". It allows you to do a few basic tasks even if your system is otherwise hosed.
Since I tend to forget the various SysRq key combinations, I like to have a printed summary of what the various keys do. Use it as you like.
Download the cheat sheet as PDF.
Download the cheat sheet as OpenOffice document.
The information in this cheat sheet was taken from the Linux kernel documentation (mostly copy-pasted) and from Wikipedia.
If you want to know more, you should read Documentation/sysrq.txt in the Linux kernel source. This file is from Linux version 2.6.22.