Okay, please, if you feel like posting interesting sites for retro game research, do it in this thread. Here's a few very interesting sites to get this started.

The Video Game Atlas database (screenshot generated maps)
http://vgmaps.com/

ZX Spectrum database
http://www.worldofspectrum.org

Commodore 64 scene database
http://www.csdb.dk

Sprite database (multiple systems)
http://spritedatabase.net/

Amiga game database Hall of Light
http://hol.abime.net/

Aminet - The original Amiga software database
http://aminet.net/tree

Commodore 64 dedicated database, games, merchandise and more.
http://www.lemon64.com/

Generation MSX - MSX computer information releases database
http://www.generation-msx.nl/

MSX.ORG - MSX database
http://www.msx.org/

Atari Mania - Atari database
http://www.atarimania.com/

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The Video Game Atlas database doesn't seem to be working any longer. Would it be alright to include this link as well?

http://www.abandonia.com/

Amazing, thanks for the post-Mr.Mouse.