Great place to find game guides, finding almost all you could want to know about game content, find cheat codes, for development utility and also for making proper protections against the codes.
is a site dedicated to unearthing and researching unused and cut content from video games. From debug menus, to unused music, graphics, enemies, or levels, many games have content never meant to be seen by anybody but the developers — or even meant for everybody, but cut due to time/budget constraints.
biggest repository of hacked roms.
Explains what hashes are. If you don't know what they are you should probably read this.
Drag and drop checksum checker utility. You can also get the checksum from the rom you have loaded from the retroachievements menu in the emulators.
An online ROM patcher utility.
Excellent for viewing raw achievements