I can't seem to figure out what this is. I know at one point I bought it at one point because I needed it for something but I can't really remember but I know it was important. It looks like a cartridge extender or something. I haven't seen it in about four years so I honestly can't remember. I tried searching on Google but I didn't have any luck. It sort of looks like an adapter but this is for the DS Lite so I don't know what it'd be adapting.
Edit: I did a search for ROM/flash cards and I can't seem to find anything thats what I have. Just R4 cards.
Edit: here's an image I found which appears to have what I have. It's apparently a ROM/homebrew thing. I do remember getting it at Gamestop so I don't know why I got it: 
And here's what I have
Best Answer
Looks to be an Action Replay. You insert it into your system, and then a game into the Action Replay. Typically allows for cheating in games and copying save files.
Gamestop often sells them, whereas I've not seen them sell flash carts.