Which authors, musicians, etc., that are still working today are geniuses in your opinion?
Hawking, for sure. Does Sarah Palin count? 
I was thinking more along the lines of arts and litertuure, but Hawkings is a genius no doubt.
In science fiction, I’d nominate Ted Chaing. Very small output, consistent critical and popular acclaim, and his stories all have ideas in them that blow you away. He has an impressive slate of awards. It’s even more impressive in that he’s only published 14 short stories.
Well, there’s me, but I’m kind of a couch potato, so I’m not sure you can call me “active.”
I’m hesitant to nominate a comic writer, but if ever there was a genius in the employ of DC and Marvel, it’s Grant Morrison.
Well, there’s…
Are you just looking for geniuses who are still working, or geniuses who are still producing genius-quality work?
I’ll take either.
Well then, in popular music, Paul McCartney and Brian Wilson are still actively producing new material, even though the work that led people to consider them geniuses was mainly done back in the sixties.
Bob Dylan certainly qualifies.
Contemporary Classical.
First, a caveat. I think it’s too early to speak of geniuses. Only time will tell whether their music sticks. History shows time and time again that some huge names still end up being considered not all that great by the following generations. With that in mind, here are some whom I’d consider contenders, listed alphabetically:
Pierre Boulez
Pascal Dusapin
György Kurtág
James MacMillan
Krzysztof Penderecki
Wolfgang Rihm
Good picks, especially Wilson. The Smile album didn’t come out that long ago. I wouldn’t be surprised if he has brilliant material he’s working on now. But he takes his time, so who knows when it will come out.
He’s not on many people’s radar now admittedly, but I’ll stick a nomination in for David Bowie.
Prince.
Jeff Beck.
Ursula Le Guin (does 2008 count as still active? I’d say so).
Beyonce, of course, cause Kanye never lies to us.
Serious answers -
Haruki Murakami.
Stephen Sondheim comes close.
Wynton Marsalis maybe?
Itzhak Perlman