Game Boy Advance

Currently the best flashcart is an EZ Flash IV.

There's the original which takes Mini SD with micro sd support via an adapter and a new version which takes micro SD natively.

It will play all games however games that require something specific may not function properly or need a patched ROM including but not limited to: Real time clock, rumble and solar panels.

Many GBA flashcarts are capable of playing DS games given the right tools. See the Nintendo DS page for more information.

Something to keep in mind with these flashcarts is that they rely on batteries to store save data, similar to older systems such as the original Game Boy. When the console is first powered on after quitting a game, the save data stored on the battery-backed memory is copied to the SD card before you are allowed to select the game you want to play. If the battery runs out, the save file is lost between power on and power off. If you are considering getting one of these cartridges, you may need to be prepared to swap the battery.

Krikzz, creator of the Everdrive line, has shown that he is currently working on a GBA flashcart. Update: that's out now.