Tool for working with your Python packages in isolation:
Docs: http://www.virtualenv.org/
Development: https://github.com/pypa/virtualenv
There is also virtualenvwrapper that extends virtualenv to make it easier to work with multiple virtualenvs:
Docs: http://www.doughellmann.com/projects/virtualenvwrapper/
Development: https://bitbucket.org/dhellmann/virtualenvwrapper
The ability is also included in Python 3.3 through PEP 405.