FlashMe is a modified NintendoDS firmware. The modified firmware removes the need for either PassMe or WifiMe to run code from Slot-2. It also allows code to be downloaded and run on the DS using WirelessMultiboot.
There have been several releases of FlashMe, If you are using a release before V5 then you need to upgrade before running any games that use the Nintendo WiFi Connection, or your DS will no longer boot.
FlashMe and a installation tutorial can be found here. If you are using a Slot-1 device you do not need to install FlashMe to run DSLinux or any other homebrew.
To Backup your firmware to a CF card using DSLinux, use the following command
cat /dev/firmware > /home/firmware.bin