What is the M3 Simply

From M3 Wiki

The M3 DS Simply is a new Slot 1 solution from the M3 Team. It uses MicroSD/Transflash memory cards (most commonly found in phones). By using Slot 1 to run NDS ROMs, the cart can run ROMs that haven't been patched the way Slot 2 carts require patching. It also means that with the use of the memory cartridge or, using a patch, a Slot 2 device (an M3 or Supercard), a user can run the Opera DS ROM with the M3 DS Simply.

The M3 DS Simply (shortened to M3 Simply) can run GBA roms in connection with the GBAexploader or the YSmenu, but you will need a Slot 2 device to do this (e.g. the EZ Flash V 3in1 Expansion Pack). The M3 DS Simply isn't compatible with SDHC MicroSD cards and the firmware developenment stopped on the 24.04.2008. There are two alternative firmwares who can be run on this card: YSmenu R4 i.L.S.