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.
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.