Wii Music (coming out Oct. 20), is Nintendo's answer to Rock Band, but is only similar to it in the sense that you're standing-up and interactively making music. Elements of competition are removed, and replaced with the ability and encouragement to play songs any way you want.
Using the Wiimote and Nunchuk controls, you can choose among a ton of instruments like piano, violin, guitar and the cat and dog (you read that correctly), and then play with your friends to make stylized versions of classic songs. The music is a bit more midi-sounding than Rock Band or Guitar Hero, the avatars just Miis, and all the songs super family-friendly, which makes Wii Music a really great option if you want to expose kids to a music game, but aren't that comfortable with the graphics and songs that the other games provide.
Check out the video to see it in action and hear what "Material Girl" would have sounded like if Madonna were a kitteh cat!