http://www.mycncuk.com/forums/showth...-DIY-Interface it's a dead project (dead for about 2 years by now). It only knows G00 and G01, and has a few other small bugs (I have use-it in the past).
http://www.planet-cnc.com/ it's an active project, relatively new (about 1 year), and works very good (I'm using it now)
I recommend the second one.
or maybe...www.usbcnc.com, but this one is not so cheap (180EUR without VAT)