There are very few programs that will allow one to paint or draw in Symmetry, which is very usefully to designers and concept artists. This is not a "How-To" in drawing symmetrically but an overview of four programs that have a symmetry tool. I added a video walk through of the applications HERE.
It has been the norm in Photoshop to draw or paint one side and then copy that side, paste and flip it to mirror the opposite side. Photoshop has the capabilities to do so much with so many various tools, it is no wonder that is is the industry standard. But for what ever reason Photoshop does not offer a simple mirror tool. Fortunately for us there are a few programs that do incorporate a mirroring tool that allows one to draw or paint in real time. So the artist can see what their design looks like as they create it.
There are two free applications and two commercial applications that I have found. Though most of my finished work is done in Photoshop, I have been looking for a complimentary program to it.
Al.chemy was the first program that I came across. It is not a painting program but a sketching program for the "creation" stage of a project. It doesn't have a brush package that one would be use to from Photoshop. Al.chemy enables you to paint with simple brushes, objects, effects, etc. Al.chemy is a great program for what it is and I still use it today. The cost is $FREE
Krita is an open-source tool as is Al.chemy but is designed for Painters on Linux systems. There are Windows builds but at the moment are a bit buggy to say the least. Krita is very similar to Photoshop for painting tools and offers a mirroring tool. However Krita's brush engine does not compare to Photoshop and with the need bug fixes is even a hard task for doing simple sketching with the mirror tool. The cost is $FREE
Corel Painter 12 is a very good compliment to Photoshop. It doesn't just offer the mirroring tool but offers a wide range of brushes not found in Photoshop due to Painter having a very different brush engine. It does a great job at simulating real media brushes, markers, pencils, oil and acrylic and even has Image Hoses which Photoshop does not have. Painter can read and write to PSD files, has it's own transform tools (not as robust as PS), can be customized to match Photoshop work environment and shortcuts. So you won't have to learn a new program from start. The mirroring tool allows for all axis and even a "kaleidoscope" effect. But for me I was only looking for a quick sketch - paint program with a mirroring tool that is relatively inexpensive (preferably free), so Painter is more than what I was looking for ...but definitely a solid program. Cost is $400 (look for deals ...they always run them)
Sketchbook Pro was the last candidate that I cam across and is a quick, resource friendly program with a decent tool set for drawing and painting. It only offers 6 layers which is fine since it is meant to replace your sketch pad. Autodesk puts out for cheap money and it will run only even some older PC's. You can mirror in horizontal, vertical or both axis similar to Painter 12. I nice added feature are the line tools, allowing for free hand strokes, straight lines, smooth consistent lines among a few. Very nice to be able to draw out a quick perspective grid on a separate layer when penning in construction lines. Cost is $59.
What I use in conjunction with Photoshop CS6 is both Al.chemy for the cool starting tools and effects (very fun to play with ...and productive) It is Free so i can have it on all of my PCs. Sketchbook for doing a lot of the beginning steps. It is great for travel since it loads quickly and doesn't use many resources and but I also keep Painter around as well since it is so much like Photoshop that I don't even really have to think about shortcuts and the brushes are a very nice difference than Photoshop.