Here are some places that you should definitely check out. If you own or know of a site that should be here, let us know on the forums!

Videogame Hacking and Homebrew

Anti-GSCentral Viper187's clone of GSCentral.com, based off a backup made on September 19th, 2004. Rather useful, despite being out of date.
ARSC Hacks Created by DsHacker14, this website contains Action Replay DS subscriptions, DS and Wii homebrew tutorials, guides and videos, Gecko Code lists (for wii) and other related things.
CodeMasters Project A site that focuses on code hacking documents. A must see if you want to catch up on current-gen hacking....
Codereversing.com - Game Hacking A very in-depth blog on reverse-engineering games. Quite an interesting read.
Data Crystal A wiki dedicated to game hacking, mostly ROM hacking at the moment. It's just starting out and doesn't have a whole lot of information, but considering it's only been around about 2 months it's not doing too bad.
Debugmo An awesome blog concerned primarily with video game hacking, but which also delves into other areas of technology. Enjoy.
GeckoCodes.org A great Wii hacking site (focused on the Gecko hacking system) started by James0x57.
HackMii A site with news and content regarding Wii hacking, homebrew, and the like. Check it out.
Kodewerx A VG hacking site started by Parasyte and a few others, including some former GSCentral.org staff. So far, it sports a fairly decent forum, and I'm guessing things will only get better. Drop by and check it out.
L. Spiro's Memory Hacking Software Home of the rather impressive (albeit generically named) Memory Hacking Software. Take a look.
Pugsy's MAME Cheat Page A page that hosts a large MAME cheat file, and also has active forums
ROMHacking.net A project with the hopes of bringing the ROM hacking community together. Check it out; they've done a good job at reviving the Whirlpool, Data Crystal, and ROMHacking.com.
The Code Hut Tony Hedstrom's site; a pretty good place concerning Game Genie and Pro Action Replay codes. Go there.
WiiBrew Center of the Wii homebrew scene. Drop by.
WiiRd blog Headquarters for WiiRD, the Wii Remote Debugger/hacking system.
XPloderFreax This site's in German, but it's got some really useful information around. misfire hails from this place, to some extent.

Emulation, Backup, and Console Modification

DCEMU Far from only concerning DreamCast, DCEmu.co.uk has a vast array (I love that phrase) of resources, including quite a bit in the PS2 dev area...
Eidolon's Inn A retro gaming site with an emphasis towards Sega related stuff. Quite useful if you're looking for certain things to go with some emulators, including the emulators themselves.
Eidolon's Inn: Kega Fusion Home of an excellent Genesis emulator, the Kega Fusion, with limited PAR hacking abilities.
GameCopyWorld Everything...well, some things...you'd wanna know about copying games, as well as a lot of other resources. Legal stuff as long as you own the original copy.
Geiger's Crypt The home of Geiger's Snes9x Debugger, and some other stuff, including a Chrono Trigger editor.
Hitmen Probably the best site for those interested in creating PSX homebrew stuff, editing a GS, or doing a myriad of other things not directly connected to PSX emulation. Go there, whether or not you understand what I'm talking about, heh.
Returner's ZSNES The site for an unofficial version of ZSNES with a few extra features, the most useful being a ram editor. Make sure to grab the English version, or you might be a bit confused.
Rveach's Site A site with no name, that has a few interesting programs. One of them is a version of the NES emulator Nintendulator that now has a debugger that can even do breakpoints on file positions, cheat search function, and a sprite viewer.
Software Laboratory The home of Dr. Hell's software, including CEP and Dr.Hell's Action Replay GPU Wrapper.
SSF Home to the Sega Saturn emulator SSF. It needs a rather powerful system, but will run most games. If you want to use it, make sure to find a BIOS.
Zophar's Domain Probably the biggest emulation site in existence. They're definitely worth checking out.

General Gaming

Castlevania Dungeon Forums A pretty massive board for Castlevania fans, with 5000+ members. Some hacking discussion takes place here as well.
Castlevania World Beyond Walls A very cool site dealing with the secrets and glitches of games of the Castlevania series. Well worth checking out.
Console Database A large collection of information about various systems and games. They've even got java-based clones of some classic games.
FF7 Citadel Another pretty in-depth fan site...don't remember why I wanted to add this one, but it probably has some cool feature somewhere, heh.
Flying Omelette This is not only host to an awesome board, but also a great Reviews section, and quite a bit more...I don't know why it took me so long to add it. I haven't dropped by there in a while, but I guess I should in the next few days. The owner's a pretty cool chick....we used to chat from time to time.
GameBase Gaming Network This looks like a pretty cool general gaming page...and you can't have too many of them. Unless you have more than twenty. That's just too many.
Nintendo Game Archive This place has all kinds of scans of NES-related stuff, including full manuals, advertisements, etc, etc. Drop by.
Secret of Mana Theater Home to some awesome SoM flash flicks, as well as a few other things of interest.
Square Gamer A pretty cool site dedicated to Squaresoft, with pics, info, and downloadable CG scenes from many of the best. They've also made some pretty cool MPGs by combining FF FMV scenes. Definitely worth checking out.
The VG Museum An awesome place all about videogames. Check it out.
The VideoGame Critic's Reviews A great place run by a great reviewer, VGC. Apparently, he plays the hardware version of every game he reviews, and wrote the review for every game on the site. Impressive place.

PC Gaming

Retro Remakes This is basically just a collection of PC remakes of classic games...they even have a Faxanadu remake! Man...it's not really complete, but it's still really cool to see a modded version of Faxanadu. Eh, it's a personal favorite.
Triple Triad Extreme: Dark Wind Triple Triad Extreme: Dark Wind (formerly Questria) is an online version of the Triple Triad games found in FF 8 & 9, only with way more features. A highly recommended download; grab it, heh.
Triple Triad Gold This is a link to the offical download page for Triple Triad Gold, a PC port of the Triple Triad card game for FF8. You definitely have to go and at least check this out; it 0wnz me every time I go back and play it.
Zelda Classic Home of the PC version of Zelda for the NES, Zelda Classic, and a bunch of modified versions...I gotta get around to really going through this place someday...

Misc. Gaming-Related

CDCovers.cc The largest collection of cover scans for almost every VG system ever, as well as quite a few music CDs. Very extensive.
Drunken Coders A great source of homebrew DS, GBA, SNES, and Wonderswan stuff.
Fenrir's Sprite Domain A place with ripped graphics from various games. There are even backgrounds here, but only as flat images...
Game Studies Bleh, just read the quote: Our mission - To explore the rich cultural genre of games; to give scholars a peer-reviewed forum for their ideas and theories; to provide an academic channel for the ongoing discussions on games and gaming.
Ludology Similar to Game Studies, Ludology is an online resource for videogame researchers, published by Gonzalo Frasca.
Non-Directional Translations A translation group specializing mostly in somewhat obscure Japanese PC games. Worth a look if you're into that kind of thing.
OverClocked Remix MP3 VG remixes! This a damn cool site that really needs no explanation.
Project 2612 A brand new and expanding colllection of VGM format Genesis and Megadrive music.
SNESAmp How about listening to your fav songs from SNES games, from a huge archive on your hard drive (3000+ songs), without taking up more than 120 megs? Check it out. SNESAmp is easy, convenient, and [email protected]$$. I definitely recommend this one.
Starmen.net The biggest Earthbound (and Mother) fan site out there. Tons of information about the games there, including music and graphics.
The Grand List Of Console Role Playing Game Clichés A list of 192 common clichés seen in many RPGs.
VGMusic Archive A huge site with thousands of midis of your favorite video games, all the way from NES to Playstation.
Which Secret Of Mana Character Are You? A little quiz thingy...fill it out for extra credit!
Zany Video Game Quotes Heh, this place is great. Everything from Faxanadu to FF9...remember that the words he's saying inside specific pages are usually links...I went a few minutes before realizing that and started out thinking the place really sucked...heheh.

Educational

Handbook of hardware pinouts, cables schemes and connectors layouts @ pinouts.ru A site with hundreds of pin diagrams for various connectors. Including some rather obscure ones rarely seen anymore.
Reverse Engineering Community Forums A great forum on reverse engineering, concerning nearly all facets of the subject.