iNES layout - a quick and dirty map of the iNES header.
iNES specifics - an indepth look at what goes where & why

System Architecture - Memory maps and register descriptions
Yoshi's System Doc - Memory maps and register descriptions.
How Nes Graphics Work - The basic theory behind NES graphics
NTSC PPU Timing - A weekend experiment and some low level PPU info
RGB Palette - [PIC] The colours you have to work with.
Sound Guide - Very detailed insight into the APU
EPROM Pinouts - Pinouts for most Eproms you can use for a homebrew
Masked-Rom Pinouts - Pinouts for Nintendos PRG and CHR chips
EPROM conversions - Short sharp explanation on how to use EEPROMs in a cart