Flashcarts are cartridges that has an SD card slot for adding your own games. They are usually unlicensed.
Flashcarts have been released for the NES, SNES, Sega Genesis, Game Gear, Nintendo 64, Game Boy/Game Boy Color, Nintendo 3DS, and (most commonly) the Nintendo DS.
List of Flashcarts
Here's a list of (almost) all of the flashcarts ever released (as of September 2014):