A popular online Mario racing game is Mario Kart 3D. This is flash-based and free to play. Mario Kart 3D uses a shopping cart for racing instead of a car. There are different race tracks and tricky turns during the game.
Another fun Mario racing game available to play online is Mario Racing Tournament. This is similar to the classic Mario Kart game where Mario and his friends race in different types of cars. First, you select your preferred character and then chose a racetrack. During each race, your character picks up objects by driving over or under the question mark boxes. Some items will give the driver a speed boost. Other items will obstruct the road or make navigating more difficult for opponents. Use the keyboard to play the game, the "W" key is for jumping and flying, "D" is to drive forward, and "Space" is to use an item.