I scratched my head for a while with this question wondering why a PS3 owner would need to ask that sort of question. Singstar has been an integral part of the Playstation stable since at least the middle of the PS2 generation. And in this generation the introduction of the singstar store with plenty of DLC in pretty much every genre describes almost exactly what you're looking for.
I did check your profile and see you are from the USA. Singstar doesn't sell nearly as well in the USA as in the rest of the world, so perhaps you've never seen or heard of it. But it is available and any decent gaming store should know about its existence even if they don't have it on the shelf.
It's almost like the question was asked with the answer already known.
I dunno if Disney's 'Sing It!' has DLC, but I think the variety of songs would be pretty limited in comparison to Singstar even if it does.
Singstar is huge fun if it's just pure singing you want.
If you can get it in the States get Buzz Quiz TV too, that is a ton of fun to play with friends when you have them over.
“The fundamental cause of the trouble is that in the modern world the stupid are cocksure while the intelligent are full of doubt.” - Bertrand Russell
"When the power of love overcomes the love of power, the world will know peace."
Jimi Hendrix
