New Super Mario Bros. Wii has four playable characters: Mario, Luigi, Toad and... another Toad.
If you're wondering why Peach isn't the fourth playable character, Nintendo's Shigeru Miyamoto recently revealed that the reason she playable is because it was difficult to animate her dress. According to Miyamoto: "we would have to have special processing and programming to handle how the skirt is handled within the gameplay."