Yeah, the "firmware" on the DS is only 256 KB in size, if I remember correctly. There's really not much room for doing much other than hacking the existing firmware, or making your own *very* minimal firmware. Thus, FlashMe is your only real choice, and it was really mostly only useful in the earlier days of DS homebrew. Nowadays you'd perhaps use it for removing the health and safety warning screen, but that's about it. My DS is flashed because I still use my slot-2 Supercard on occasion, and I like being able to boot it into DS mode without my Superkey.
As The Catboy said, FWNitro was another custom firmware back in the day, and the only other one I can remember was called CreeboMe which I would not recommend even trying.