If you are talking about 2d apps, there is "project dogwaffle" which is a very powerful (and free) graphics program. sorry i dont have the link but do a google search and it will turn up something. If you want 3d, then for a starter wings3d would be the way to go, I've seen a 10 year old get the hang of it in just a little while, and even with its advanced functions anyone can play around with it. Im sure there are many more apps i dont know about, but best of luck!