Useful Links

http://www.videogameconsolelibrary.com/index.html - Huge compilation of every single console ever. Very informative and it has great pictures as well.

http://www.retroactive.co.nz/ A New Zealand based retro gaming site, run by FeedbackBsr on SA. Covering anything and everything from 1999 earlier it's New/Videos/Reviews/Editorials galore!

http://www.atariage.com/ - Atari Age, all about classic Atari systems

http://bataribasic.com/index.html - Batari BASIC, a BASIC compiler for Atari 2600 development

http://cgfm2.emuviews.com/ - Console hardware info and modifications

http://famicomworld.com/ - Famicom World, all about the Nintendo Family Computer and its accessories

http://www.hardcoregaming101.net/ - Hardcore Gaming 101, one of the greatest sites on the planet for info about all kinds of older game series. Lots of obscure info here. HIGHLY recommended reading.

http://www.multimods.com/ - TurbografX/PC Engine/Duo modifications and repair

http://www.nesreproductions.com/ - NES Reproduction carts at reasonable prices

http://www.raphnet.net/electronique/electronique_en.php - A shitload of hardware modifications, really cool. Also features the best guides on disabling an NES' Nin10 lockout chip and repairing the 72-pin connector(don't buy a new one if yours is fixable)

http://www.nesplayer.com/ - Nintendo Player, NES game reviews and editorials. Pretty decent read.

http://www.racketboy.com/ - Racketboy shouldn't need an introduction if you're interested in retrogaming. Highly recommended.

http://retr0bright.wikispaces.com/ - Retr0bright, a recipe for a plastic brightening solution that will make old plastic electronic cases look brand new.

http://www.retrowaretv.com/home/ - Retroware TV, a lot of web shows with reviews, retrospectives and other general retrogaming info. There's some crap, but a lot of good stuff.

http://www.retrousb.com/index.php - Everyone knows about Retrozone

http://www-unix.ecs.umass.edu/~dhowland/mod/ - Sega Genesis hardware mods, probably work on the Megadrive as well

http://www.sega-16.com/Home.php - Sega 16, all about the Genesis/MD and really really good.

http://www.smstributes.co.uk/ - Sega 8-bit, pretty much the Sega Master System version of Sega 16

http://smspower.org/ - SMS Power!, another Master System fansite, with information on foreign releases and obscure hardware

http://www.nesworld.com/ - NES World, the oldest NES fansite on the internet (started in 1993) and still updated frequently.

http://nesdev.parodius.com/ - NES Dev, THE go-to site for NES game development and hardware technical info.

http://bobrost.com/nes/ - Online version of a college course on NES game development, an incredible resource for getting started on programming for the ol' toaster.

http://www.nesicide.com/ - ...and to help you along, this is an NES-specific IDE

and finally,

http://kevtris.org/ - Bluetech, Kevin Horton's homepage. Dude is to old game system hacking as Tiger Woods is to fucking waitresses. Except better.