No work stands alone, and we have great admiration and gratitude for the many excellent websites dedicated to the 6502, Atari 2600, microprocessors, emulation, and simulation.  Here, we list just a few that have been particulary helpful and interesting. This is by no means complete, and we'll be expanding this section as time allows.

- 6502 - - central hub of all things 6502, thanks to Mike Naberezny
    MOS 6502 hardware manual, 1976,   6502 Programmer's manual, 1976
    6502 documents
    Forum thread for our site
6502 on Wikipedia
6502 History - article about MOS Technology and the 6502
6502 and the Best Layout Guy in the World
Western Design Center - Keeping the 6502 alive in CMOS
6502 'illegal' opcodes - discussed at
Beregnyei Balázs' schematic - in Hungarian, 6502 abbreviated schematic
    English translation - partial

- Atari -
    2600 programming
    TIA schematics - original design docs
    STELLA Programmer's Guide - reformatted from an original doc
    TIA programming thread - excellent article by Andrew Davie
Original Atari chip design tapes - Curt Vendel recovered design files
     AtariAge forum
Design of 'Adventure' - the first action-adventure video game, by Warren Robinett
    SIGGRAPH '96 PowerPoint slides - Warren Robinett

- Commodore, C64 -

MOS 6581/8580 SID die shots - MOS sound interface device,
MOS 6567/6569 VIC - MOS video controller and the Commodore 64

- Apple -

Apple II FPGA - Stephen Edwards reconstruction

- Microprocessors in General -

Intel 4004 35th Anniversary Project - Tim McNemey,
    Lajos Kintli's simulator - excellent visual tools! - Erik Klein's wonderful site
The CPU Shack Museum - John Culver's vast collection of chips
Recollections of Early Chip Development at Intel - Andrew Volk, Peter Stoll, Paul Metrovich, 2001
Making of a Chip - Intel, 2009
Companies - and their great blog (No affiliation to us) - de-capping, x-ray, photography (we haven't used them)
Circuits - small collection of more modern gates
    Jeri Ellsworth - who builds chips at home

- Simulation & Emulation -

Simplicity Betrayed - George Phillips, 2010
Stella - Atari 2600 emulator