The DSMEM is a device created by Amadeus, designed specifically to run DSLinux.
It has very good support for userspace XIP, and was once the device providing the largest amount of free RAM for DSLinux to use. It uses a microSD card (also known as "Transflash") as a storage device. It is also optimised for speed. The dslinux.nds image is flashed into fast flash memory and runs from there. The DSMEM is probably still the device that runs DSLinux the fastest.
The DSMEM has no built-in RAM, and has thus been superseded by support for RAM in the SuperCard and M3. Very few people got one, and Amadeus will probably not sell anymore of it.
The development of the DSMEM is documented in a thread on the forums, which is probably the longest thread in history of the DSLinux forums.