if you are looking for a zbrush clone, this program is still in alpha stage a little buggy, but you can still use it:
http://www.drpetter.se/project_sculpt.html
This other program takes another aproach to paint your textures:
http://www.drpetter.se/project_cherrybrush.html
This next is a page of a lot of programs for people that are more inclined to 2d drawing, that means, you draw in 2d and the program will create a 3d shape, there are different ones and for different purpuses, for drawing for painting, etc, most of them are alpha stage too, but they are all fun to play with. Good thing about these ones is that they don't write anything in your registry, you unzip the folder anywhere you want on your computer.
http://www-ui.is.s.u-tokyo.ac.jp/~takeo/research/Projects.html
this next one is interesting because you use when you already have different meshes and you want to mix them, if you have different meshes you can create some interesting designs with this one.
http://www.meshmixer.com/index.html
Hope you have fun with these programs.
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