Multiple Arcade Machine Emulator (MAME) Source Code
Arcade Restoration - Extensive database of PCB Information, Part/ROM Lists, and Memory Maps extracted from MAME Source Code
Arcade Museum / Killer List of Video Games - Largest database of arcade games
SYSTEM 16 and [ARCHIVED] Wikipedia's List of Arcade System Boards - Hardware info on arcade systems
Home of the Z80 CPU - Nearly everything about the Zilog Z80 and "close relatives"
[ARCHIVED] Z80 Bits (of information) - Some Z80 Routines
Eeeems' Tutorial Hosting - Z80 Tutorial, References, and Routine
ClrHome - References on Z80 Assembly (and an online assembler) and TI-BASIC. Best Z80 instruction set visualization!
GP2000 - Z80 Assembler and TRS-80 Family of Computers Emulator, Software and information on both
6502.org - Nearly everything about the 6502 processor (and related)
The Real Bob Roberts - Extensive repair logs, references, and and other information (and part shop)
Multigame.Com - Good reference materials, including custom Namco IC part numbers
NESDev (Wiki, Databse, and Forum) - Maybe relevant to Nintendo VS.
SMS Power! - Extensive resources on the Sega Master System and other Z80 CPU based Sega systems (maybe relevant to the Sega Mega-Tech system)
Red Cedar Electronics - Great page on the Votrax SC-01A Speech Synthesizer chip
Video Game Music Preservation Foundation Wiki
mdfs.net - Software and documentation related to low-level programming and early computers, including the Z80 and 6502 processors
Mike McBike (all in German) - Information (mainly PCB images) and repair logs on several arcade games and early home computers
Euro-Arcade - Arcade Part Shop (although last update was in 2005) and Information (good Pac-Man technical overview)
Fix Your Pac-Man and Fix a Galaga
JAMMArcade.net (Blog run by multiple people, see their personal pages in Links section) - Extensive resources and replacement parts for repairing arcade games
Vintage Gaming and More - Atari 2600/7800 Mods, Kits, and Information (related by 6502 and PIA)
Retro Computing (Hans Otten) - Some info on the 6809, 6502, and Z80 based computers
HeinPragt.nl (in Dutch and English) - "Retro computer info"
Chris Kilgour's Personal Projects Page - Great articles on the Namco 52xx and 54xx chips and Pole Position!
Progetto-Snaps: The Italian MAME's Resource Place - Collection of MAME resources (other than ROMs) and resource lists going back to 1997
The Guru's ROM Dumping News (New site as of August 2nd, 2023)
Arcade Game Typography (Book)
HEROTALK - A Text-To-Phoneme Translator For The Votrax SC-01 Speech Synthesizer
Custom Resolution Utility - Helpful for outputting video signal from computer to CRT
Museum of Soviet Coin-Op Machines (in Russian and English) - 2 locations in Moscow, 1 in St. Petersburg
Multiple Arcade Machine Emulator (MAME) - See source code link for technical information