I tend to use a lot of software that is a bit on the pricey side to create my artwork, yet there are still a lot of very useful programs that i use from time to time that are free. Here are a few.
Sculptris
Is a free sculpting program that allows for 3D molding, its an alternative to Zbrush. While it doesn't have as many options as Zbrush it still manages to be quite a powerful tool when it come to creating faces, characters, and basic renders. http://www.pixologic.com/zbrush/downloadcenter/
Blender
This one is a difficult program to learn like a lot of 3D software. Though with a little practice one can start to creating very detailed bas