That's funny, other than a few hotbutton issues, McCain and Obama are pretty much identical policy-wise. I don't think its a case of extremes, but rather the convergence of both parties to about the same place resulting in rhetoric full of sound and fury; signifying nothing.
Its kind of the way really big mutual funds tend to end up mimicing the general stock market. You've got two huge parties that account for roughly 99% of the votes - when they are that broad-based they both end up at the same place. So they have to make a huge deal of the very minor differences between them.
Slightly extreme would be someone like Ron Paul or Ralph Nader. Really extreme I can't even name any politicians that fit the bill on the right, just talking heads like Pat Robertson or Michelle Malkin vs somebody like Brian Moore of the Socialist Party USA (despite his being way more capitalist than someone like Karl Marx).