All flash carts can run MoonShell, a very good MP3/OGG player that also does video. I have a Datel Games n' Music (slot-1), it's a pretty low quality device, but it runs MoonShell fine. If you want better, the R4, M3Simply, SuperCard One, etc will all run well. I would avoid Slot2 devices because you have to use a PassMe (slot-1 device that passes program execution to slot-2). The DS doesn't natively run DS code from the 2nd slot, so it requires a sort of hack to make it do so. However, I also have a GBA Movie Player V2 (upgraded with unofficial firmware) and with the modified firmware, it can run DS code from Slot-2, but I have to use my Games n' Music to boot it.
I have some videos about DS Homebrew on Youtube, you might want to check it out:
YouTube - Broadcast Yourself.