The most common (and affordable) programs that FPSC users work with are Blender, Milkshape, Google Sketchup, Fragmotion, GIMP, and paint.net (I'm sure I'm forgetting a few others). Blender has a sculpting tool similar to Zbrush (but not as advanced), and Blender is free.
EDIT: There really isn't a "best" choice out of those programs because all of them get the job done, and different users here prefer one over the other. I can tell you that out of those three modeling programs, Blender is more advanced than Milkshape, which is more advanced than Sketchup. GIMP and Paint.net are about the same, and Fragmotion is great for animating, but some users prefer using Milkshape for animation.
Kravenwolf