For a C2D system, Orthos works nicely for stress testing. It will automatically run two instances to load both cores.
Prime95 is a little harder to set up.
You start it, select Blend or Small FFT or Large FFT in the popup which comes up, then click Start.
It generates a worker for each core automatically. No setup required.
Just remember it runs in the background if you close the window. Always use 'Stop All Workers' from the menu.
+1 for Prime95.