I'm looking into LinuxCNC.org and their EMC2 software. It's free. Otherwise, the centroid people have a lot of retrofits out there. Don't know how good they are, though. Same with Fagor, but going into more money.
beege im currently experimenting with linux emc2. currently its beating everything functionality wise and cost, just got to properly install linux on my lap to and get it configured up for the mill. then make some chips. but so far my only complaint is you cant edit code in the run window. whilst i have used geko drives and have nothing but good things to say about them there g rex controller only seams to be readly supported by mach software. i want to try and keep my options open and not get tied into a specific supplier for anything. whilst this some times costs a bit performance wise, spares and reliability seam to go up whilst maintinance costs drop.