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 [[http://www.python.org/dev/peps/pep-0405/|PEP 405]].