From a hobbyist point of view there a few out there.
In no order there are Desk CNC, Turbo CNC, Mach 3 , CNC pro and EMC.
This isn't a full list by any means but its a list of well supported programs.

To go thru them quickly in the same order as they are stated.

Desk CNC is reliant on a serial board from the same company and support for this isn't too good in the UK.

Turbo CNC is well though off, it's still in DOS but the download isn't crippled in any way although they do ask you to register this.

Mach 3 is probably one of the best out there, over 10,000 registered users and a fantastic support group both web based and on Yahoo. Very powerful and even beats some industrial controls. The demo is limited to 100 lines of G code


CNC Pro is now in the public domain and is no longer supported but still has a good following. Not all G codes supported but most of the popular ones are.

EMC runs under Linux and is free but unless you are very Linux savvy it can be a long learning curve.

There are others but most seem to be very limited versions of art type programs.

John S.