Check out the Jello Tetris. There are instructions for making Tetrominos from Jello. We used the knife method when I was little, but now there are cookie cutters and ice cube trays. I strongly recommend using the Jigglers variation with extra gelatin if you plan to make Jello shapes. Note that Jello will stick to itself unless you oil it, which is relevant if you want to arrange the pieces so they actually look like a Tetris game, let alone move them. You can use unflavored cooking oil, or if your pieces are savory in flavor, use a more interesting salad oil such as black walnut.