X-bit Labs released a guide about voltmodding power supplies.
There are two systems components that traditionally undergo volt-modding: mainboard (Vcore, Vdimm, sometimes Vagp, Vdd, Vio, and certain model-specific modifications) and graphics card (Vgpu, Vmem, sometimes Vref, Vddq, OVP). Besides these two, you can also volt-mod the Power Supply Unit (PSU) of your computer. A simple modification allows controlling the basic voltages (3.3v, 5v and 12v) of a majority of existing PSUs.
Few off-the-shelf PSUs offer you such control over the voltages. I could only name certain hi-end units from Antec (like the TrueControl 550), PC Power & Cooling, OCZ Technology and SOYO. However, even such units would only become better from some additional tweaking, as their standard voltage ranges are rather narrow.
