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Another suggestion: folding@home with InCrease...

There is even a Mac OS X team.
 
I know there are a few scientific and number crunching apps that will do this.

From a graphics/rendering perspective, the only stuff I am familar with is Handbrake for DVD encoding/transcoding and Adobe After Effects CS5 for complex video effects rendering. Both of these Applications will take advantage of all physical cores plus all of the virtual cores in hyperthreading.
 
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