For those that are less electronically inclined, take a look at the Super-PID at
www.superpid.comGives 5,000rpm up to 30,000rpm from any universal (brushed) AC motor. Has optical feedback to maintain precise RPM control. The $4 optional PCB allow Mach3's PWM output to control rpm. This will be a built in function on V2, which will be available soon. It also gives you on/off control from Mach3, with no additional hardware required.
In testing on a Porter Cable 690, I've found the actual rpm to be within 50rpm of the commanded RPM in Mach3 over the entire usable range, 5,000-27,000 for the PC 690.
I've got it wired directly to a Xylotex drive. It's so nice to be able to use the Spindle Override slider in Mach3 with a regular handheld router.