Some links with information about programming languages that are useful when programming the kernel. When adding links, please group them by category or from most basic to most advanced.
C
Wikipedia's entry on the C programming language
CProgramming.com Tutorials on C and C++
Introduction to C A long web page on C
Zed's Learn C the Hard Way Alpha release
C Cheat Sheet (pdf) - actually a lot longer than one sheet, it's like a more detailed version of a quick reference card
Open-Standard Standard Latest draft standard for C
Assembler
Programming from the Ground up Downloadable PDF
Intel Assembler 80x86 CodeTable an overview of the x86 assembler instructions, on one sheet of paper
if the above page is down, try this cache
Ralf Brown's Interrupt List exhaustive info about just about every interrupt for the x86/DOS environment, especially useful for the detailed info about BIOS calls here
Sandpile.org hardware and software information about x86 and x86-64 CPUs
Perl
Rex Swain's HTMLified Perl 5 Reference Guide - much more detailed than the cards
Python
Other links
GRUB From the Ground Up - a tutorial/introduction
How to Debug Programs - an introduction
How To Write Unmaintainable Code is a collection of excellent rules for writing code that nobody else can understand. It is an excellent article on how NOT to write programs