As somebody has mentioned, you could use ITK, which could be used with VTK as a visualization toolkit. The only problem you will find is they are not very easy to build if you are going to use their python wrappers (and even more difficult if you try to build the third-party open source project wrapITK).
But you could use python(x,y), which comes with all these tools (and a lot more) running out-of-the-box and in python:
http://www.pythonxy.com
Hope it helps.
