I don't know about software, but there's a wonderful and long thread here in the forums that's called "HUNDREDS OF TUTORIALS..." telling where to find videos and instructions for lots of decorating techniques. Since most are on CC, YouTube and places like that, they're yours to consult for free.
Marianna "I know my own mind...and it's around here somewhere!"
There are some sites that have design software to play with that really doesnt give you much. I dont know about any other free software but there is Wedding Cake Design Pro that you can buy. The website is under the same name.
I am actually in the process of developing both the design software and mangement system. I can tell you from researching, there is only one Cake Design software that I found and it is redicuously expensive. Like the others have said, look for the tutorials, they are wonderful.
Go to getpaint.net. While it is not designed for cakes, it is a good photo/art program. The internet is loaded with clipart. You can make your own and it will not be a :cookie cutter" cake. And it is Free.