Any controller that will accept an incremental encoder input can be used. I am using Galil Motion Control products, but there are a lot less expensive controllers out there that will work just fine. They can be installed as a retrofit on almost any machine. The magnetic read heads are about $90 each for 1 micron resolution, and the magnetic tape is about $50/M. Not terribly expensive, less money I think than glass optical scales, and much more compact.
I have these installed on my 48x96 router, my 10x50 mill, and just installed DRO on my 13x40 lathe using mag scales. Have been very happy with the result.
Here is a picture of the X scale on my mill. Sorry for the blurry picture.
http://www.cnczone.com/forums/attach...d=360446&stc=1