OpenCNC is also a brand name for a specific CNC control package based on PCs running Windows. They claim to be less expensive and more flexible than the conventional CNC controls (ALlen-Bradley, Fanuc, et.al.) as well as the PLC running behind them.
I've had OpenCNC quote a retrofit for one of my more obscure machines, and I'm seriously considering it. Can't give you any feedback from a real-world experience, though. The one concern I remember having was that it was not compatible with the latest versions of Windows, but I could be wrong about this.
Now, I'm going to have to look into the Siemens package that Biffi mentioned.
I look forward to the rest of the replies.