there are a few, prob one came with your mainboard on the cd. but i dont know of any that i have seen actually work. they usually just hang your system.
It allows you to muck with the multipler and voltage in windows, so long as you disable Cool 'n Quiet.
Keep in mind that newer Athlon 64 chips only allow software to set the voltage as low as 1.1v. You can set it even lower, but you'll have to go to the BIOS for that.
My laptop's Sempron Mobile allows me to go as low as .8 V with RMClock. Of course it's only stable at that voltage when I'm running it at the lowest speed of 800 Mhz. 1.1 V is stable at 1.6 Ghz though. It's much better than the 1.45 V and I barely hear the fan.
I tried cpuid and it works fine in my old pc but my new one won't change in windows although maby my bios has some tampor resistance or something, I dunno but it will let me change mutliplier in windows but not voltage.