Goomba (emulator)
Goomba, named after an iconic enemy in the Mario series, is a freely available Game Boy emulator for the Game Boy Advance, developed by Fredrik Olsson (FluBBA). When the project was first started, The Only Reason for development was for the sheer fun of it, since the GBA could already play GB games natively. After Nintendo DS and Game Boy Micro were released, Goomba has had a boost in speed optimizations and features since neither of them work with GB games.
Development of the original Goomba seems to be ceased, however since the source code is available in the public domain, an unofficial version has been forked with Game Boy Color support, named Goomba Color, by Dan Weiss (Dwedit). While most games play with great compatibility, some heavy programs slow down in Goomba Color, and changing color palettes in the middle of a frame rendering is not supported (some GBC games use this trick to simulate "high-color" modes).
However, while FluBBa may not have had a good excuse to create this emulator, it filled an important need. GBA Flash cards cannot natively play Gameboy or Gameboy Color games, even on the original Gameboy Advance. This is due to a difference in voltage used by the cartridges. The legacy hardware included with the Gameboy Advance in order to read and play Gameboy and Gameboy Color games can only be used when an actual cartridge is inserted. Since GBA Flash cards are read as Gameboy Advance cartridges, this sector of hardware is locked out, making it impossible to run a Gameboy or Gameboy Color ROM natively, So, for all practical purposes, Goomba ironically enables original Gameboy and Gameboy Color games to be played when they otherwise couldn't be, through software emulation, the same way that any other system, such as the NES or Game Gear, can be emulated on a Flash card. Furthermore, because the Game Boy Micro was designed without the legacy hardware at all, emulation through Goomba becomes the only way to play original Game Boy or Game Boy Color games on a Game Boy Micro. Various versions of Goomba now come standard with almost all GBA flash cards.