Shaun White Snowboarding is really fun on the balance board. I also like Rayman Raving Rabbids TV Party, but it's not ALL on the board and it is more of a minigame party (though it is fun and hilarious).
As workouts go, My Fitness Coach is very good, but does not use the board. It hardly even uses the remote! But, it does provide a great workout. Jillian Michaels Workout Ultimatum uses the board but gets TERRIBLE reviews so I'd skip it if I were you. EA Sports Active will be out soon, so stay tuned...
We Ski uses the board, and I think its now combined with Snowboarding for the same MSRP. Although I personally prefer playing the game just using the remote and nunchuk.
Its not much of a workout game though.