From vgmrips

Here we have a list of online game databases, with info, screenshots, cheats and other stuff related to each game they list. These are useful for getting info on packs that are being submitted (though you may want to try corroborating from many sources), and for including external links in the pack info.

Site Games Search API Offline Info
General Databases
VGMdb Yes Yes Large, soundtrack-oriented games database
The Cutting Room Floor Yes Info on unused content in games; excellent source of release dates, publishers/devs, and hidden sound tests/songs
Video Game Music Preservation Foundation 1466 Yes Seeks to be the Wikipedia of video game music.
Please note that their guide to VGM logging is severely outdated despite recent page edits, and you should use R(er)ipping for the Out-of-Element Contributor instead.
Internet Movie Database Yes Despite the name, also contains info on games and their music authors/soundtracks/OSTs, including uncredited music sources... assuming these are accurate
mobygames Yes Large, general DB
giantbomb Yes Yes Large, general DB
GameFAQs Yes
thegamesdb Yes Yes Large, general DB
gamesdbase Yes looks like a large DB, contains some rare games
Video Game Den Competent-looking game reviews for a handful of systems
GameBase No No Yes Collections of Games+Music+Screenshots+Extras, and an Access database for each system. Data seems ripped from other places for some of the GameBases. See GameBase.
allgame (archived) An old database that got closed down. kept some of it though.
refuge tokyo (previously mercenary force) Google No Yes Many systems
RF Generation Yes No Many systems
visual novel database Yes Visual novels.
Computer Emu Zone Spanish (but also english and other languages), various systems (Spectrum, CPC, C64, MSX/MSX2, PCW, Atari ST etc.)
mamedb Yes Arcade
Arcade History Yes Yes Big Arcade MAME DB
arcade museum Yes arcade games
Sega consoles
SMS Power Yes Wiki format, info about lots of SMS games
Guardiana Yes Most, if not all Sega home consoles
Sega Retro Yes Wiki format, info about lots of Sega games
ZX Spectrum
World of Spectrum 24000 Yes Spectrum games + downloads
NintendoNerds Yes Database with Nintendo games, including NES and SNES
NesCartDB 2411 Yes Yes Database with Nintendo cartridge info, with technical details
superfamicom Yes SNES and NES databases
famicom world game list Yes Famicom listing
gbdb No No GameBoy; Seems fan-made but it looks alright
TurboGrafx 16 / PC Engine
PCEngine Catalog Project 954 Yes Old-school
PC Engine Software Bible No PC Engine listings
PCE Daisakusen 884 Yes PC Engine database
Generation-MSX 4773 Yes MSX games
msx zone No MSX games and other resources
MSX Game World Yes MSX games with roms
Hall of Light 5631 Yes Yes Amiga games
8-bits PC-8801/SR 1127 Yes 8-bits - Japanese database
8-bits MSX 285
8-bits MSX 2/2+/R 62
8-bits X68000 672 1356 Yes Deals with Shoot-em-ups for many systems, in french
Vectrex Museum VGDB 191 Vectrex games
The Internet Pinball Machine Database 5937 Yes No No Pinball machines, also features music rips