A wonderful thing that every games console should have. Since Europe uses the PAL TV system (50Hz), and America and Japan use NTSC (60Hz), PAL games consoles must run 20% slower than their NTSC counterparts. This majorly annoys everyone in Europe, and persuaded them to find ways of making their consoles run at the propper speed. In some systems (such as the original Mega Drive, Sega Saturn, and Neo-Geo) the speed is set by a block of jumpers, so the change can be made simply by cutting one link, and making another. Others, such as the SNES, and Mega Drive 2 require that a pin on a chip be lifted, and either tied to ground or +5V. Once this mod is made, you have a fully functional 60Hz console. However, Nintendo UK, being the bastards that they are, decided it would be fun to make games detect the speed at which the console ran, and refuse to work if all was not as expected. To counter this, the mod is made reversable by means of a switch, often mounted on the consoles casing. (In all fairness, it wasn't just Nintendo that had this bright idea. Mega Drive games by Konami, and some by Sega have this nice "feature".)

Consoles of recent years (eg Sony PlayStation, and Sega Dreamcast) use software switches to choose the speed. Therefore, the speed at which your system runs is determined by the games. This is why it's such a good idea to by all PS games on import. Sega have been nice enough however, to include a 60Hz mode as a selectable option in many (but not all unfortunately) UK Dreamcast games, lending them some extra favour with gamers.

See also: RGB mod and lock-out bypass.

As a source for these mods (until I steal some of 'em), Game Station X http://www.gamesx.com/ is highly recomended.