Happy Wheels is a gory disturbing game by Jim Bonacci. It is entirely free to play, and can only be played in a browser. You don't even have to register in order to play, but doing so gives you the added benefit of being able to save your replays. And you will want to save some of those.
The game is fairly easy to pick up and play. The controls are shown under the game window, and they are very simple. Since the game heavily relies on user-generated content every map can have a different goal. Most of them do have a goal area you must reach, while the way to get there can be everything from driving in a straight line to not being able to reach it by design. While the game has a timer on each map it doesn't follow the normal racing-game mentality most of the time. Some maps are made with getting the best time in mind, but mostly the game is about the damage you can inflict on your driver. Jim Bonacci has also made a sick damage system in the game, making the game a lot more interesting than other racing games in the same style. Limbs will snap off, splatter into gibs and hang from your body by tendons alone. Your character will function through all this, but will die if the spine is injured heavily, or if the head is injured. The system is more or less realistic, and makes for some disturbing gameplay as your character cries out in pain.