Best Way to detect click fraud

What companies or tools do you use to click fraud?

[JerryOdom]

So how do you detect fraud when its happening from Google or Yahoo? Or how about other PPC Engines like Looksmart or MIVA?

What companies or tools do you use? Sticky or post here pleeeeeeeeeaaase!

Thanks,
Jerry

[JerryOdom]

Nobody here uses one of the click fraud detection companies or systems?