Page15 about Mach3: Unfortunately, the software is currently for Windows only, so Linux users will have to beg, borrow, or steal a machine outfitted with the OS until it's been converted.
You don't need to change the OS. For Linux there is LinuxCNC (originally known as EMC2, enhanced machine controller). It's based on a realtime extenstion to the Linux kernel (Xenomai) for the critical parts and a GUI. It also includes a ladder-logic PLC to automate parts of a machine. The G-Code preview is rendered using OpenGL.
There is a quite strong community around LinuxCNC (as with Mach3 as well), both are used very successfully to retrofit old machines (with broken controllers or to upgrade manual machines).
The author has covered many good examples of freeware but obviously cannot list all of them. KiCAD is already fairly well known, as is LTspice. I think the author's intent is to list not-so-well-known freeware, and I applaud his efforts.
Readers' comments as to other good examples of freeware are very useful too, and add informative detail to the original article.