is it possible to use mach 3 M3 spindle control to turn on and off a porter cable router ?, if so how would you do it ?, use a relay ?, if so what kind,  has anyone done this ?

Not the cheapest way, but the best way is to get a Super PID, which also gives you rpm control in Mach3.
www.superpid.com

Super cheapest (in stark contrast to the above!  ha ha) would be to tie a transistor onto a pin (just assign that pin as the spindle control), have the transistor fire a relay on/off.

CNC4PC has several relay board options, as well as the relay itself.
http://www.cnc4pc.com/Store/osc/product_info.php?cPath=24&products_id=518

http://www.cnc4pc.com/Store/osc/product_info.php?cPath=24&products_id=357

I am trying to switch my spindle on/off through pin 1, output 1. The problem is that whatever I do the output 1 is allways low. When I switch off the computer it get high. I measured pin 1 of the parallel port and I allways get 4,something Volts idependent of the spindle button in mach 3 is on or off. Is that a problem with the parallel port? or is my breakoutboard that is defective? I have a probotix breakoutboard PBX-RF. Can anyone help?