I use CIMCO V5.0 for some editing and it is nice and very taylored to CNC programming. But…
For you who like to tinker and have something different ( and free !!! ) Check out NotePad++
I use it for HTML / CSS as well as G-Code. You can customize to your hearts content, and the file search ability allows you to search quickly through files for specific text. Handy when you are trying to track down a program by searching for a part number in the comment section of the program.