Nintendo Power (cartridge)
Nintendo Power was a service in Japan that ran from September 30, 1997 to February 28, 2007 and allowed players to download Super Famicom games on to special, white flash memory cartridges known as SF Memory Cassettes. At a later date, players were able to download Game Boy games on to white GB Memory Cartridges. The cartridges were manufactured in Taiwan. 
Following the relative success of the Famicom Disk Writer, the service was intended to be a cheaper alternative to purchasing the full retail game.
While the majority of games available for the service were existing retail games, some games were exclusive to the service. For example, Picross NP was a series of eight Super Famicom Picross games based on popular Nintendo franchises. While Super Mario Bros. Deluxe was previously available overseas, it was only available in Japan via the Nintendo Power flash RAM service. 
Purchasing and downloading games
Nintendo Power cartridges were originally sold over the counter of Lawson convenience stores based in Japan. Although the cartridges were also sold at the Japanese supermarket Daiei, re-writing services at stores other than Lawson were not formally introduced.
A user would first purchase a SF Memory Cassette or GB Memory Cartridge, then bring it to a store which had an NP copier for the Loppi ticketing system.
The player would select games to be placed on the cartridge, and then had them loaded on. In addition, the store would provide the purchaser with a printed copy of the manual for the game and blank stickers. The player was expected to write on the stickers and place them on the cartridge to mark what games they had downloaded. Game prices varied, with older titles being relatively cheap, and newer titles and NP exclusives being more expensive.
Purchasing pre-written versions of Nintendo Power games was relatively cheaper than buying a blank cartridge and manually downloading games. An exception is the Deluxe Pack version of Fire Emblem: Thracia 776, that featured special merchandise and was retailed for ¥9800.
Beginning of the Game Boy service
The Game Boy re-writing service was originally scheduled to begin in all Lawson stores from November 1, 1999. However, the impact of Taiwan's 921 earthquake in September 1999 meant that it became increasingly difficult to produce a stable supply of GB Memory cartridges. This meant that the beginning of the Game Boy service was postponed until March 1, 2000.
In October 2000, as an apology for the service being postponed, players could order a pre-written version of Wario Land: Super Mario Land 3. This was a promotional item that was not re-sold. In later years, it was only possible to obtain Wario Land: Super Mario Land 3 through the Nintendo Power service by re-writing it on to an existing cartridge.
Termination of the service
As of August 31, 2001 the Nintendo Power service was partly terminated and it was no longer possible to purchase Nintendo Power cartridges at Lawson or use the Loppi ticketing system to download games.  However, from September 1, 2001 players could directly send used cartridges to Nintendo through customer services to have them replaced with a different game.
As of February 28, 2007 the service was fully closed.
Each cartridge's flash RAM is divided internally into eight blocks. Unless an 8-block game is loaded onto the cartridge, however, one block is reserved for the game selection menu, leaving only seven blocks for games. In addition, each cartridge has a small amount of SRAM for game saves, which is divided into sixteen blocks. Games are rounded up in capacity (i.e. a 10 megabit Super Famicom game needs three flash RAM blocks (12 megabits), a Game Boy game that needs 100 kilobits of save space would need 2 SRAM blocks (128 kilobits).) The system does have one limitation - games that utilize a special chip (such as the Super FX) cannot be placed on the NP cartridge, as the needed chip was not in the cartridge.
Mario Games released on the Nintendo Power Service
SF Memory Cassette
Along with rereleases of hit Super Famicom games, several Nintendo Power titles were released exclusively for the service. The Super Famicom version of Dr. Mario is the same as the one found in the compilation Tetris & Dr. Mario, which was not released in Japan. Wrecking Crew '98 was first available on the Nintendo Power service and was later released as a regular retail version.
GB Memory Cartridge