Here is a nifty little poll ranking public intellectuals--you can vote for those on Prospect Magazine/Foreign Policy's Hot 100, or add your own.
My list:
Noam Chomsky (on the left)
Christopher Hitchens (formerly on the left, now mostly on the right)
Niall Fergueson (firmly on the right, but interesting)
Umberto Eco (what are his politics?--I like his lit crit and novels)
Fareed Zacharia (typically on the right, but very critical of the neo-cons of late)
And my "Bonus Ball" addition:
Howard Zinn (on the left)
My criteria? If I've read them and been challenged by what I've read--and if I admired them whether or not I disagreed--they made the list (though my admiration for Hitchens fades fast). Altnernates would include Zizek and Boudrillard and Coetzee and Naomi Klein. An altnerate "Bonus Ball" is Amartya Sen.