The Rumble Pak for the Nintendo DS is an official DS Option Pak shaped like a Game Boy Advance cartridge and is designed to be inserted in the system's Game Boy Advance game slot (SLOT-2). It was the first Nintendo DS accessory to utilize the slot. In North America, it was first made available bundled with Metroid Prime Pinball, but is now available through other promotions or as a stand-alone purchase from Nintendo's online store. The accessory was released differently in Europe due to a delay of Metroid Prime Pinball in that region. Various DS games supporting the Rumble Pak (Metroid Prime Hunters, Mario & Luigi: Partners in Time, and 42 All-Time Classics) had been released in Europe before the accessory was available. The accessory was eventually released as a pack-in with the game Actionloop in the UK.
Although the Rumble Pak works with both the Nintendo DS and the redesigned Nintendo DS Lite, it protrudes from the Nintendo DS Lite when inserted, as do Game Boy Advance cartridges, because of the Nintendo DS Lite's smaller size. For this reason, a smaller version of the Rumble Pak is available in Japan, which is flush with the system when inserted. It was initially available in just black, but other colors produced by unlicensed accessory manufacturer eWin, such as white (see picture), have been released. Other color variations been released by eWin.
The Nintendo DS Rumble Pak is incompatible with the Nintendo DSi and Nintendo DSi XL, as both consoles lack a SLOT-2. However, none of the games supporting the Rumble Pak require it for proper operation of the game, meaning that the games themselves are still compatible with DSi (some software titles, such as Nintendo DS Browser and Guitar Hero: On Tour, have SLOT-2 peripherals that are required for operation, and are hence incompatible with DSi and DSi XL, although the former already has an alternative version made specifically for the Nintendo DSi and later consoles). Despite Nintendo DSi and Nintendo DSi XL's lack of SLOT-2, some Nintendo DS titles released post Nintendo DSi's introduction continue supporting the Rumble Pak accessory, mainly due to the fact the Nintendo DS Lite console is still being sold concurrently to the Nintendo DSi and Nintendo DSi XL.
Source: Wikipedia, "List of Nintendo DS Rumble Pak games", available under the CC-BY-SA License.