There used to be the R4, cycloDS evolution, M3 DS real, Acekard 2, and many others for the DS/DSlite. However, with the DSi released, you need a new cartridge to play downloaded, FREE roms/games.
These are all top-of-the-line DSi flash carts. No "fake R4" bull, or anything like that. These are the best on the market, and the prices inckude shipping fees and paypal fees! DSi Flash Carts:
Acekard 2i - $30
iTouch2 - $35
EzFlash Vi - $30
DS/DSlite Flash Carts:
CycloDS Evolution - $50
M3 DS Real - $40
DS-X - $30 (you'll look awesome with this)
N-Card - $10 (really cheap, but limited games. Great for really young kids who just want to play mario kart and such!)
DSlink - $10 (flashed DSlite only)
Shipping is free, as stated above. Pictures of the DSi Carts after the cut!