Sudokuhax is an exploit for the game Sudoku by EA, was only available through the American and European/Australian DSi Shop, for 200 Nintendo Points. The exploit works by hacking the game's savedata to make it to load the file "boot.nds" from the SD in DSi-mode. Sudoku was removed from the DSi Shop and Nintendo/EA is in the process of patching it, see the hackmii post.
More instructions and download is available here: Hackmii post