ReiTech (www.reitech.com) makes several flavors of this to fit just about any shop tool. It is more than just an ON/OFF switch - it has a magnetic contactor in it that will shut-down the power if you have a electrical outage, too, so the machinery won't re-start on it's own when the power is restored.
They also offer machine bases with the EASY OFF fully integrated upon which you can mount your existing tools. This is great for those items that are sitting on the wobbly handmade bench just waiting to topple-over.
For other E-Stop Options, look at: