I agree Muzzy is only a good introduction for young kids. If you can sample it for free at your library even better.
I have used Rosetta Stone with ds, but it was hard to keep his interest. I may try again though since it does seem comprehensive. Another thing to try for free that is very similar to Rosetta Stone is Livemocha.com. However, you need to sit there with your child to supervise since occasionally pop messages will occur from other users asking you if you want to converse. I just close the pop boxes. I have never seen anything objectionable though on the site:)
Another possibly is Mangolanguages which I have not used but I have heard good reviews.
