There are a plethora of free software programs to do this, including (but not limited to):
MRIcro, MRIcron, MRIcroGL, Slicer 3D, ImageJ (Fuji), mrview, fslview, freeview, ITKsnap. There are also (as pointed out above) many on github and nitrc
A bit late to this discussion, but there seem to be too many options, each with its own learing curve and hassles. Which is the best for 3D looking scientific illustrations of cortex and connector white matter? E.g. Fiber tracks surrounded by a transparent cortex showing Broca and Wernicker areas. Perfer easy interface with little effort if possible.