Developer The Binary Mill has been trying a number of different approaches on iOS (including the Penny Arcade-approved Decide-o-Tron game recommendation app), but Mini Motor Racing HD is the company's most solid release yet, in my estimation. It's a top-down mini car-racing game with a pretty exceptional amount of polish. The controls are well-made, there's a lot of fun progression options for a number of different cars, and the cars themselves are really well-rendered, so much so that on the iPad especially they really do look like real little toys.
The game only has one option for multiplayer, you can only play with one other player, either over WiFi or Bluetooth. But the single-player campaign mode is pretty filled out anyway, with more than 20 tracks and plenty of cars to unlock and upgrade. There's Game Center integration for leaderboards as well, so there's plenty of game to play through.
Mini Motor Racing HD is a solid mini racing game -- the cars feel solid, handle well, and the tracks are short and fun enough that the game's enjoyable even when you don't come in first. It's available on the iPad right now for $3.99 or on the iPhone for $1.99.