I used the standard tileset from Minecraft for the tetriminos, so graphics partially made by mojang. I just coded it in a few days. I do not intend to make any money off this project.
Oh, and coïncidentally, someone else around the time I made this had about the same idea as me. You can find his prettier and probably more interesting version right here: http://www.minecraftforum.net/topic/1353751-version-115-selfmade-java-minigame-craftris-the-minecraft-tetris-dismember-shoot-burn-or-blow-up-those-mobs-click-play/